telemetry: Use trace categories to configure synthetic delays

BUG=307841
TEST=tools/perf/run_benchmark scheduler.tough_scheduling_cases

Review URL: https://codereview.chromium.org/103143007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243993 0039d316-1c4b-4281-b951-d872f2087c98
Adds the new URLRequest::OnBeforeNetworkStart hook to the
ResourceThrottle so that the ResourceScheduler can eventually throttle
closer to the start of network usage.

This is the second step in the design doc:
https://docs.google.com/document/d/1TSI3jwozVB_nueWJxzi8uKbgDfsGZRZqw8tvtD-P1Iw/edit?usp=sharing

The third step is to create the new ResourceScheduler.

BUG=328741

Review URL: https://codereview.chromium.org/129173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243994 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243993
SVN changes up to revision 243994