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On each user action, the client now can receive the custom data map.
This CL adds the custom data to impression proto. We will plumb this
data to the client and add this data to impression tracker in a
following CL.

Bug: 977255
Change-Id: I4b9552bf30c620a66d466cf6eeabc0f2549666fe
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717703
Commit-Queue: Xing Liu <xingliu@chromium.org>
Reviewed-by: Hesen Zhang <hesen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683413}