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This patch first splits the test into two to test UMA / UKM separately.

For UMA test, it
  - adds a run loop to make sure the JavaScript has been executed.
  - adds a call to |FetchHistogramsFromChildProcesses| to make sure that
  the necessary render process could be initialized before fetching the
  histograms. crrev.com/c/1804574 updated the functionality of this
  helper function.

Bug: 1141862
Change-Id: Id119b2b01d0f46faab56cd0739e78a5fbeadf2c7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566250
Commit-Queue: Yi Gu <yigu@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832537}