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This patch adds two entreis to rappor.xml for Web Components usage.
Blink CL is at:
https://codereview.chromium.org/986583002/
Chromium CL is at:
https://codereview.chromium.org/1014543003/

BUG=461671

Review URL: https://codereview.chromium.org/1071463003

Cr-Commit-Position: refs/heads/master@{#324344}