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Due to my previous CL (crrev.com/c/1849096), |first_tappable_index_|
and |last_tappable_index_| are calculated in the wrong way. This CL
fixes such an issue.

Bug: 1013811
Change-Id: I2723070b8a0ecfef31e82433148d1df84937e23f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1857043
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705788}