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Record the size of ad resources according to MIME type. This will help us
understand which resources are using up more network bytes and allow
us to better estimate the effect any ad intervention will have. Only
HTML/JS/CSS/Image/Video are recorded as they are expected to capture most
of the resources commonly seen in ads. It is expected that these metrics
will not be completely accurate because requests do not always contain
the correct content type. It is assumed that the MIME type in the response
header is a suitable proxy resource type.

Bug: 881049
Change-Id: I000466fb3fe5467c4f73527cb4cedbd7709d7c3f
Reviewed-on: https://chromium-review.googlesource.com/1219900
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593087}