GitLab

This change:
1) Introduces color_utils::ImplicitAlphaBlend, which does an alpha
   blend using the alpha of the foreground color;
2) Introduces a NativeTheme color ID for the shading of a pressed
   button;
3) Uses ImplicitAlphaBlend to blend that shade, instead of calling
   into PlatformStyle;
4) Deletes PlatformStyle::BackgroundColorForMdButton

BUG=644543

Review-Url: https://codereview.chromium.org/2319313003
Cr-Commit-Position: refs/heads/master@{#417366}