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This ensures that the devtools_page URL has correct scheme and host,
both when loading the manifest and when pushing data to DevTools front-end.

Bug: 1059577
Change-Id: I69a7ccdccfae31781ead371a85d23df36f108665
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2118894
Commit-Queue: Andrey Kosyakov <caseq@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753369}