[webui][ntp] Perf traces for applying theme, loading background image

Adds traces to measure the duration until the theme is set and the
background image is loaded. Since the background image is loaded in an
iframe, the load event is sent back to the NTP via a postMessage and
logged there. If we logged the background image load time in the iframe,
the event would be logged with respect to the iframe's load time and,
thus, wouldn't be comparable to other NTP traces.

Bug: 1061078
Change-Id: Iaf5ceec2c6ee44e0f838cf25ae2c1cbbe0f03a58
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2181522
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Esmael Elmoslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#765387}