Update comments in GetMaxRendererProcessCount().

Better explain how RenderProcessHost::GetMaxRendererProcessCount() works
and fix some nits in the code.

BUG=104689

Change-Id: I28d0bc664e5416b67dd15a6ada109ef136e49665
Reviewed-on: https://chromium-review.googlesource.com/1087755
Reviewed-by: Charlie Reis <creis@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#565165}
1 file changed