[Telemetry] Only calculate tab contents bounding box once.
[Telemetry] Only calculate tab contents bounding box once.

Cache it and use the first calculated box. This helps Speed Index be more stable
and should avoid some asserts we were seeing.

BUG=

Review URL: https://codereview.chromium.org/163023003

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@252476 0039d316-1c4b-4281-b951-d872f2087c98

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