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Avoid having to define Log classes in every layer needing them.

The previous usage of Log required layers and components to
redefine Log classes and have them declare log instances. This was
not convenient and made base packages needlessly aware of specialized
layers.

This approach has the callers specify the group tag at every call, removing the need to get any sort if logger instance.

BUG=472152

Review URL: https://codereview.chromium.org/1080543002

Cr-Commit-Position: refs/heads/master@{#325487}