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This moves the package name sampling result into a boolean member
variable (which slightly improves performance, and is more consistent
with is_in_sample_), and rephrases a comment.

This fixes some tests to mock the IsInPackageNameSample() value prior to
MaybeStartMetrics, rather than after, since the value is cached during
that method.

This is a follow up to feedback on https://crrev.com/c/1898711.

Test: run_android_webview_unittests --gtest_filter=AwMetricsServiceClientTest.*
Change-Id: I824540aa7362ab34f9cb4981b88cc446338bdff3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1937704Reviewed-by: Tao Bai <michaelbai@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719429}