Use a canvas to check pixels in offscreen canvas printing tests

Recent mojo changes have caused flakiness in ofscreen canvas printing
tests because the test can finish before the canvas has been able to
draw to the screen. Instead of asking the offscreen canvas for a pixel
directly, draw that canvas to an onscreen one and test that for
completion.

We're still testing the ability of an offscreen canvas to print, the on-
screen version is only used to solve a race condition.

Bug: 921719
Change-Id: I2725c309a9878f7ba74d126f03a7d2a1f39b435b
Reviewed-on: https://chromium-review.googlesource.com/c/1470526
Commit-Queue: Aaron Krajeski <aaronhk@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631827}
3 files changed