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During a rotation, when BrowserFragment is retained and the containing
activity is recreated, the following sequence of events happens:
1. destroyAttachmentState
2. createAttachmentState
3. onFragmentStart <- when this is called isAttachedToWindow() is false
4. onFragmentResume <- when this is called isAttachedToWindow() is false

BrowserImpl maintained some state to avoid doing certain
things during a configuration change, but that state was reset
in onFragmentStart(). The problem is onFragmentStart()
triggers updating the visibility of the WebContents, and at
the time onFragmentStart() is called isAttachedToWindow() returns
false, meaning the WebContents was hidden.

Hiding the WebContents is bad for sites like youtube which
stop playing video.

Then fix is to delay resetting state until onViewAttachedToWindow().

There is one other subtle change to TabImpl. Specifically
getViewController() is changed to return null if the
BrowserViewController hasn't been updated yet. This is necessary
because the code updates the active state of the tabs *and* then
the BrowserViewController. This means if when updating the active
state of a Tab the Tab calls back to BrowserViewController the
state isn't right, and may cause NPEs.

BUG=1075744
TEST=TabTest.testRotationDoesntChangeVisibility

Change-Id: I14e68251541f4387338e5548d4d78daac7419935
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2439556Reviewed-by: Bo <boliu@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#813197}