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This CL fixes two issues:
1. transitioned based on edge_color_ for foreground animation. Previously, we
transition based on center_color_.
2. use alpha channel in the SkColor correctly. Previously, the alpha channel
in SkColor didn't apply to its RGB channel before blending.

Bug: NONE
Change-Id: I3533c062d632412cd181d0ed89c770910e6d56e3
Reviewed-on: https://chromium-review.googlesource.com/747444
Commit-Queue: Biao She <bshe@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513122}