Configure synthetic delays through TraceLog

Make it possible to configure synthetic delays using trace categories.
For example, the category filter "DELAY(gpu.SwapBuffers;16)" would make
swap buffers take at least 16 ms to complete.
    
BUG=307841

Review URL: https://codereview.chromium.org/98953002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244207 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244207