Move BidiCharacterRun into the rendering partition

This CL moves BidiCharacterRun into the rendering partition so that it can go
fast. Previously, only BidiRun (a subclass of BidiCharacterRun) was in the
rendering partition.

This issue was discovered by profiling the infinite scroll case from the Silk
benchmark.

R=esprehn@chromium.org

Review URL: https://codereview.chromium.org/136003017

git-svn-id: svn://svn.chromium.org/blink/trunk@166000 bbb929c8-8fbe-4397-9dbb-9b2b20218538