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This forbids production code depending on these targets, and enables
these targets to depend on other testonly targets in the future.

The only dependency prevented core:testing from being testonly was
from BlinkLeakDetector.
Add Page::SetInternalSettingsPrepareForLeakDetectionCallback()
to reverse the dependency.

Change-Id: I64cba73f0c4d1383a192e468220f3d55e926a8f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864720
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708516}