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The clip is inset by the separator opacities, which means the favicon bounds
animate as the separator fades.  It's possible crossfading would look better,
but it's not clear (and that's harder to implement).

This also makes a few other minor changes:
* Use kSeparatorThickness in tab_strip.cc for consistency
* Try to improve comments
* Use a lambda when computing separator colors to avoid repetition
* Ignore subsequent tab hover value for leading separator.  This is technically
  more correct; in practice it would only be visible if the bounds animation
  were slow enough that you could have a preceding tab still animating back into
  position (thus not covering the leading separator) while hovering a subsequent
  tab.  Since that's not really possible, this is mostly just to make the code
  easier to reason about.

Bug: none
Change-Id: I37eceae6c3b12a3730a0706853fc8cc1526c16c4
Reviewed-on: https://chromium-review.googlesource.com/1121785
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Allen Bauer <kylixrd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572067}