The GN configs for different optimization levels are confusing and
should probably be reworked, but for now we add a special config
specifically so that we can compile with -O3 when appropriate.

R=brettw@chromium.org, machenbach@chromium.org
BUG=616031, 618678, 621335

Review-Url: https://codereview.chromium.org/2076403002
Cr-Commit-Position: refs/heads/master@{#37158}