[Start] Fix: Tab strip is showing on the Start surface at cold start.

The bug was caused when the TabGroupUiMediator is created after the
overview mode (Start surface homepage) is showing. Thus, adding the
overview mode observer will miss the first showing event, and the tab
strip won't hide. In this CL, we check the overview mode in the
constructor of the TabGroupUiMediator, and update the
|mIsShowingOverViewMode| immediately before any call of
resetTabStripWithRelatedTabsForId() happens.

Bug: 1239272
Change-Id: I8f76fdcd473f0fe77077a7fd3f4d6da55f8b3f3d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3098165
Commit-Queue: Xi Han <hanxi@chromium.org>
Reviewed-by: Mei Liang <meiliang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#912318}