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This CL adds logging for a sparse UMA histogram Tabs.TabOffsetOfSwitch to record
how many tabs forward or backward users move when switching tabs in the Android
tab switcher. This will help us measure usability in experiments.

Bug: 828224
Change-Id: Id7e5bb2431b30408688dbfeb02ee762e8c3facf5
Reviewed-on: https://chromium-review.googlesource.com/1050936
Commit-Queue: Ryan Landay <rlanday@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557335}