Add two crash keys before GPU watchdog kicks in.

One is how long ago ReportProgress() is called. This gives us a hint if we need
to call it more often.

Second is the available physical memory. This indicates if GPU delay is caused
by page swap, etc.

BUG=597700
TEST=none
R=piman@chromium.org

Change-Id: I18d87b2fcdaf061a388c7da4d763b303c6197013
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1525412
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#641423}