feat: disable fetching thumbnails if thumbnailSize is 0 (#14906)

Capturing window thmubnails is expensive as it actually uses the
window capturer and it records one full frame per window and then
downscale to the default size 150x150. When only interested in the
window names or the app icons we do not need all of this.

Underlying change is merged in chromium72 so this patch only modifies
the doc, see:
  https://chromium.googlesource.com/chromium/src.git/+log/72.0.3626.52/chrome/browser/media/webrtc/native_desktop_media_list.cc

Example: desktopCapturer.getSources({thumbnailSize: {width: 0, height: 0}}, ...)

Also added a unit test in spec/api-desktop-capturer-spec.js that verifies
that the returned thumbails are of type NativeImage and empty,
when the user disable fetching thumbnails.

notes: Can disable fetching the thumbnails for the DesktopCapturer.

https://github.com/electron/electron/issues/14872

docs/api/desktop-capturer.md

@@ -82,7 +82,9 @@ The `desktopCapturer` module has the following methods:
   * `types` String[] - An array of Strings that lists the types of desktop sources
     to be captured, available types are `screen` and `window`.
   * `thumbnailSize` [Size](structures/size.md) (optional) - The size that the media source thumbnail
-    should be scaled to. Default is `150` x `150`.
+    should be scaled to. Default is `150` x `150`. Set width or height to 0 when you do not need
+    the thumbnails. This will save the processing time required for capturing the content of each
+    window and screen.
   * `fetchWindowIcons` Boolean (optional) - Set to true to enable fetching window icons. The default
     value is false. When false the appIcon property of the sources return null. Same if a source has
     the type screen.
@@ -107,7 +109,9 @@ captured.
   * `types` String[] - An array of Strings that lists the types of desktop sources
     to be captured, available types are `screen` and `window`.
   * `thumbnailSize` [Size](structures/size.md) (optional) - The size that the media source thumbnail
-    should be scaled to. Default is `150` x `150`.
+    should be scaled to. Default is `150` x `150`. Set width or height to 0 when you do not need
+    the thumbnails. This will save the processing time required for capturing the content of each
+    window and screen.
   * `fetchWindowIcons` Boolean (optional) - Set to true to enable fetching window icons. The default
     value is false. When false the appIcon property of the sources return null. Same if a source has
     the type screen.

spec/api-desktop-capturer-spec.js

@@ -4,6 +4,7 @@ const chaiAsPromised = require('chai-as-promised')
 const { desktopCapturer, ipcRenderer, remote } = require('electron')
 const { screen } = remote
 const features = process.atomBinding('features')
+const { emittedOnce } = require('./events-helpers')
 
 const { expect } = chai
 chai.use(dirtyChai)
@@ -105,4 +106,20 @@ describe('desktopCapturer', () => {
       expect(sources).to.be.empty()
     })
   })
+
+  it('disabling thumbnail should return empty images', async () => {
+    const { BrowserWindow } = remote
+    const w = new BrowserWindow({ show: false, width: 200, height: 200 })
+    const wShown = emittedOnce(w, 'show')
+    w.show()
+    await wShown
+
+    const sources = await desktopCapturer.getSources({ types: ['window', 'screen'], thumbnailSize: { width: 0, height: 0 } })
+    w.destroy()
+    expect(sources).to.be.an('array').that.is.not.empty()
+    for (const { thumbnail: thumbnailImage } of sources) {
+      expect(thumbnailImage).to.be.a('NativeImage')
+      expect(thumbnailImage.isEmpty()).to.be.true()
+    }
+  })
 })