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* Add a GN argument to whether show a launcher icon to open WebView
  developer UI main. This gn argument is off by default.
* The shown icon will be another icon in addition to any icons added by
  WebView providers.
* A launcher activity is defined as an activity-alias that links to
  developer UI main activity.
* The manifest file where this activity-alias is defined is merged into
  WebView manifest only if the flag is true.

Bug: 989831
Test: Manually inspect the new added launcher icon after installing system_webview_apk, monochrome_apk, trichrome with gn arg 'webview_devui_show_icon=true`
Change-Id: I9e5b808f5e1f9491d2ed813ccf342207a7642b9f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863929
Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Auto-Submit: Hazem Ashmawy <hazems@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711391}