Add MotionMark to third_party library

Since the MotionMark repository is in WebKit itself, which makes it
hard for chrome to do add, modify or remove tests. We added
MotionMark to third_party/blink/perf_tests directory. In this way,
we can continue contribute to MotionMark project.

Note that this is a temporary solution, this directory will be removed
when MotionMark becomes a public project itself. This adds around
1.3MB to the project.

Github URL: https://github.com/WebKit/webkit/tree/master/PerformanceTests
Revision: 4b28b98d37306eb5ae9ab8bca504590295f64210

Bug: 1446401

Change-Id: I040a8497bdedffab0cb7b0f369c0a389ff41b92c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4541141


Reviewed-by: Rick Byers <rbyers@chromium.org>
Commit-Queue: Yi Xu <yiyix@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1148691}