This moves the pixel testing browser test utility functions added in
plugin_power_saver_browsertest into the common browser test utilities.

The generalized version now reuses functions used in cc pixel testing.
We also replace the test-specific rebaseline flags with a more generally
usable --rebaseline-pixel-tests flag in base/switches.h.

Bug: 956675
Change-Id: I16080b9fe94e100e4d93ecdd364fecb507526a15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1610844
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Tommy Li <tommycli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#787556}