telemetry: Switch ImageDecoding measurement to tracing.

PERF SHERRIFS: This patch resurrects the benchmark. Numbers on the dashboard
may jump from zero (or not reported) to their actual values.

Fix ImageDecoding benchmark and move it to tracing. The existing version was
broken, because the inspector timeline recording was started in
DidNavigateToPage, when the benchmarking has already completed. The results
were therefore empty and no data was reported. This patch starts the recording
in WillNavigateToPage. It also switches the measurement from inspector timeline
to tracing. The reported numbers are the same (within the noise band). The
measurement works with and without imple-side-painting (impl-side is not forced
by the measurement).

R=nduca@chromium.org, tonyg@chromium.org
BUG=324032

Review URL: https://codereview.chromium.org/185153002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254386 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254386