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- Show launcher icon for Standalone WebView and Trichrome WebView
  prestable channels (dev and canary) but not for Monochrome.

- The GN flag still has to be enabled for local builds to show the icon
  since they have `android_channel=default`.

- Use a different activity-alias for Monochrome which is disabled by
  default i.e the launcher icon for monochrome is disabled by default
  and the GN arg will not show the icon for monochrome till
  crrev.com/c/2000724 is merged.

Bug: 1026062
Test: Set android_channel="dev", verify standalone WebView and TrichromeWebView show the icon
Test: Set android_channel="dev", verify Monochrome does not show the icon
Test: Set android_channel="default", verify no WebView icons show up
Change-Id: Icd218fac086aa940bca61f2ac6ef3ceea76cf0db
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993817Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Cr-Commit-Position: refs/heads/master@{#733563}