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- This CL changes the back button when a user is in a WebAPK from using
Activity#finish() to using Activity#finishAndRemoveTask()
- Activity#finishAndRemoveTask() is still used if the user pressed back
while the splash screen is up because WebApkActivity is still translucent
at this point and Activity#finish() causes an ugly transition when
WebApkActivity is translucent.

BUG=982956

Change-Id: Ie39b2c6902bef2d22e6052a47013f15975fbb611
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717556
Commit-Queue: Peter Kotwicz <pkotwicz@chromium.org>
Reviewed-by: Pavel Shmakov <pshmakov@chromium.org>
Reviewed-by: Dominick Ng <dominickn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#681846}