[ MimeHandlerView ] Add a new UMA bucket

This CL adds one more bucket to MimeHandlerViewUMATypes which counts the usage
of internal calls (from C++) to postMessage (used for printing).

TBR=wjmaclean@chromium.org

Bug: 659750
Change-Id: I08c5194012c97f0a603dd338e454b201dfafd1da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1576204
Reviewed-by: Ehsan Karamad <ekaramad@chromium.org>
Commit-Queue: Ehsan Karamad <ekaramad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652748}
3 files changed