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This CL migrates the legacy InitiateCaptureDump IPC message
to the new Mojo defined in WebTestClient interface.

Bug: 1039247
Change-Id: Idd38ecf67d10593f3bfc6f3c48d9832514cec595
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032634
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Mike West <mkwst@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739222}