[GRC] Report first alert fired UMA for background tabs.

This patch adds UMA histogram to report first alert fired happen for
background tabs, histogram records the time from when the tab becomes
background to when a JavaScript alert is fired.

Minor fix: Remove entry from web_contents_data_map_ when
WebContentsCoordinationUnit is destroyed.

BUG=731270, 756004

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_site_isolation
Change-Id: If04a495aef60d7e300177271547a472792da5c3b
Reviewed-on: https://chromium-review.googlesource.com/614913
Commit-Queue: lpy <lpy@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Zhen Wang <zhenw@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496113}
9 files changed