Make compositing update 2% faster

Previously, we were spending 2% of the compositing update in Silk's
toggle_drawer allocating memory in the overlap map's vector. By adding some
inline capacity, we can avoid the malloc in the common case.

The large effect on the profile seems like it ought to be a symptom of a larger
problem with how we're using the overlap map, but we added some counters and we
seem to be adding layers to the overlap map a linear number of times.

Review URL: https://codereview.chromium.org/179773006

git-svn-id: svn://svn.chromium.org/blink/trunk@167858 bbb929c8-8fbe-4397-9dbb-9b2b20218538