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. README added for representative perf tests.
. Update pixel-wrangling instructions to include looking at
  repr_perf_test results.
. Linked the pixel-wrangling doc to the representative perf test's for
  more detail.

Bug: chromium:1000837
Change-Id: Iefe03f7c722ccc9c8d72cf3e5ada7816e240d1cc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937848Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Behdad Bakhshinategh <behdadb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#720547}