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Tab helpers are attached to prerender web contents, and as a result
TabLoadTracker counts these web contentses as tabs. This CL adds
additional tracking of non-UI tabs and adds Get*TabUiCount() methods
that exclude non-UI tabs from counts.

This CL also modifies CoreTabHelperDelegates to notify TabLoadTracker
when tab contents are swapped. In that case, non-UI tabs are promoted to
UI tabs.

Bug: 861800
Change-Id: I3666b17634efd1445cd24cade8514fee06ad2c92
Reviewed-on: https://chromium-review.googlesource.com/1134504Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Scott Haseley <shaseley@google.com>
Cr-Commit-Position: refs/heads/master@{#577886}