Add a new folder for pixel testing.

This will be used to support pixel test framework for desktop ui.
Summary: the framework can be used for browser_test to write pixel tests.
The image diff webui is provided by Skia Gold.
Design doc(Internal):
https://docs.google.com/document/d/1Mz72RHWyQKsT6jynrDNGbCaTKQIy_77qMjRM5gpsr_w/edit?usp=sharing
Design doc(Public):
https://docs.google.com/document/d/1uvXp0AKfemJ9e1TXM1nH3UTcVlDKISK2jFd_g2gstX0/edit?usp=sharing

Bug: 958242
Change-Id: I2991597ed82621710555ccb2fac59b2e88511e6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1589685
Commit-Queue: Sven Zheng <svenzheng@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657417}

chrome/test/pixel/OWNERS

+svenzheng@chromium.org
+bsheedy@chromium.org
+
+# COMPONENT: Test>Pixel
+