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- Track thumbnails only if they are currently in view or are
just a standard finger swipe away.
- Do not track thumbnails for pinned tabs.
- Do not track thumbnails when the tab strip is closed.

Note that this CL does not implement actually untracking thumbnails.
It merely calls the method in the C++ side that will eventually do it.

Bug: 1015132
Change-Id: I16cce4a71ad3e6e887555be27e1525cdef9beb33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894245Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: John Lee <johntlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711609}