Set default font size for OffscreenCanvas to 10px

There's no hard standard on this, so we are matching the behavior
of detached canvas.

Bug: 977876
Change-Id: I84f8e32000f290d6826722863bb6fb16116e2f18
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1675911
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Yi Xu <yiyix@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672198}
8 files changed