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WebView uses an unspecified platform mechanism to upload UMA logs,
rather than the typical cronet path. This creates a WebView-specific
MetricsLogUploader, which sends logs to PlatformServiceBridge, which
connects to the platform mechanism.

BUG=698047

Review-Url: https://codereview.chromium.org/2778203003
Cr-Commit-Position: refs/heads/master@{#460649}