Dialogs don't drop tab fullscreen when in FullscreenWithinTab mode.

Overrides the default behavior of dropping fullscreen when a tab modal
dialog is opened in the FullscreenWithinTab case. This is because, in
FWT mode, the browser window is in its normal layout (not fullscreened).

Bug: 883535,812769
Change-Id: I1c262954b962d508eb86ef9a8a312bec03ab2332
Reviewed-on: https://chromium-review.googlesource.com/1228976
Commit-Queue: Yuri Wiitala <miu@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592522}
3 files changed