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This is an attempted relanding of
https://codereview.chromium.org/1348833003, which was reverted in
https://codereview.chromium.org/1450083002 . This cl includes changes
to address the problem in https://crbug.com/555576 .

It turns out that DecrementCapturerCount() can trigger a call to
content::WebContentsImpl::WasHidden(), which eventually calls
ThumbnailTabHelper::UpdateThumbnailIfNecessary(), starting the
thumbnail generation process all over again. It also appears that the
bitmap capture operation is not necessarily synchronous, so calling
DecrementCapturerCount() anywhere within ProcessCapturedBitmap()
potentially disposes of the web contents before the thumbnail has been
captured.

This new cl differs from the original (1348833003) only in
thumbnail_tab_helper.cc, where I now unregister the ThumbnailTabHelper
from further notifications within UpdateThumbnailIfNecessary(), and
call DecrementCapturerCount() once the final callback is completely
finished with the web contents.

BUG=530707

Review URL: https://codereview.chromium.org/1461463002

Cr-Commit-Position: refs/heads/master@{#370809}