Create ResourceTimingInfo in willStartLoadingResource()

FontResource defers calling FontResource::load() until the resource
is actually needed. This makes ResourceTimingInfo's initial time
inaccurate. This CL is a workaround which waits to create ResourceTimingInfo until ResourceFetcher::willStartLoadingResource() is called.

BUG=323750

Review URL: https://codereview.chromium.org/246443003

git-svn-id: svn://svn.chromium.org/blink/trunk@172432 bbb929c8-8fbe-4397-9dbb-9b2b20218538