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Verify that request headers specify a passthrough when "Save Image As..."
feature is used with Data Saver enabled, and this header is not specified
with Data Saver disabled.

Override SaveFrameWithHeaders in TestWebContents to keep track of
the headers.

BUG=467163

Review URL: https://codereview.chromium.org/1137563002

Cr-Commit-Position: refs/heads/master@{#329900}