Move SaturatedArithmetic from Blink to base

This is a more efficient implementation than what gfx is doing in most
cases.  This CL moves it to base, updates coding style, and switches
gfx to using it.

TBR=thakis@chromium.org
BUG=659381

Review-Url: https://codereview.chromium.org/2499783002
Cr-Commit-Position: refs/heads/master@{#433474}
19 files changed