Remove the report load timing field in ResourceRequest

Since this field is always set to true before being used only in WebURLLoaderImpl, this CL removes it entirely.

BUG=376025

Review URL: https://codereview.chromium.org/316353004

git-svn-id: svn://svn.chromium.org/blink/trunk@176109 bbb929c8-8fbe-4397-9dbb-9b2b20218538