GitLab

This add methods to compositor_util.cc to compute the number of
raster threads to be used in the renderer process. Then this number
is passed to the renderer process explicitly instead of just forwarding
a command line flag blindly.

If the renderer will use more than one thread, chrome://gpu will report
that "Multiple Raster Threads" is enabled, otherwise it shows disabled.
If the --num-raster-threads command line argument is used to force more
than one thread, then it will show Force enabled.

There is no change in behaviour with this patch, it still uses one thread
unless forced otherwise on the command line.

BUG=237669

Review URL: https://codereview.chromium.org/519923002

Cr-Commit-Position: refs/heads/master@{#292726}