Add feature flag for fullscreen alert bubble.

This CL add a feature flag kFullscreenAlertBubble to the feature
of Fullscreen Alert Bubble.

Bug: 1107185
Change-Id: I76c3ef33df6056f7a33f0a23294ce2ef169a2326
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2520323


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824911}