[WebView] Correct metric for state save time

This metric had two problems:

1. It wasn't measuring correctly for ANGLE - in that case, state saving
happens within ANGLE when the context is made current.
2. It was recording in milliseconds rather than microseconds.

This CL corrects both, updating the metric name.

OBSOLETE_HISTOGRAM[Android.WebView.Gfx.SaveHWUIStateDuration]=Superceded by corrected version

Bug: 1168587
Change-Id: Idf4d3e062348b546d20d3f294285d048a265835c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4555541


Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148490}