GitLab

In single process mode with impl-side-painting, browser and renderer
would both try to set number of raster worker threads, which causes
DCHECK. This CL makes sure there won't be duplicate call to set number
of worker threads in single process mode.

R=enne
BUG=314185

Review URL: https://codereview.chromium.org/971943002

Cr-Commit-Position: refs/heads/master@{#318995}