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This increases the sample rate for pre-stable channels (the stable
channel sample rate is still 2%). This change is not expected to have a
significant impact on total UMA volume, due to the relatively small size
of WebView's pre-stable channels.

This addresses privacy concerns (regarding fingerprint-ability for
low-usage applications) by limiting the package name field to 10% of the
population. When sample rate < 10%, we'll upload package name with all
UMA logs (this is the status quo). When sample rate > 10%, we'll only
upload package name for the clients which fall within a 10% group (using
similar logic to our existing sampling logic).

This continues to maintain the requirement that we only upload the
package name for system apps or Play Store-installed apps (ex. This will
not log the package name for sideloaded apps, even if the client falls
within the 10% package-name-logging group).

Bug: 969803
Test: run_android_webview_unittests --gtest_filter=AwMetricsServiceClientTest.*
Test: build with android_channel="beta", manually observe metrics are logged 99% of the time
Test: build with android_channel="stable", manually observe metrics are logged 2% of the time
Change-Id: I611c76b21f790251ab08de3f72cd0807f9a88f9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898711
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712879}