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This reverts commit 18d1b898.

Reason for revert: Test ConversionsUseCounterBrowsertest.ImpressionClicked_FeatureRecorded consistently failing on Linux Asan Lsan bot: 
https://ci.chromium.org/p/chromium/builders/ci/Linux%20ASan%20LSan%20Tests%20%281%29/80021

Original change's description:
> Add UseCounters for Conversion Measurement API
> 
> Adds Blink Usecounters for impression registration, conversion
> registration, and a composite usecounter for both of those events.
> These are necessary to measure the percentage of page loads using the
> API, for running an origin trial.
> 
> A new test harness is added in the chrome/ layer to verify that
> metrics are recorded as expected.
> 
> Bug: 1109079
> Change-Id: I2f70bb3a2eac60ff45a5654169b4db959fd1d3f4
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2314843
> Commit-Queue: John Delaney <johnidel@chromium.org>
> Reviewed-by: Nate Chapin <japhet@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Reviewed-by: Charlie Harrison <csharrison@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#792856}

TBR=thakis@chromium.org,japhet@chromium.org,csharrison@chromium.org,johnidel@chromium.org

Change-Id: I93dc0b42b6684777dff0106300ef64d00366fd42
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1109079
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2326796Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793031}