Dynamic module: Add custom header.

For module managed's URLs, add the header: X-CCT-Module-Client-Data to
the main resource requests.

This is disabled by default, implemented behind the flag:
CCTModuleCustomRequestHeader

Design doc:
https://docs.google.com/document/d/1wX9jnp43URSHOcHkfN4IX6PbW4M7TYWT8P8xOeYkQtQ

Bug: 900594.
Change-Id: I10a0d9f9e664de1fb1571ab5e87550bcefc752c3
Reviewed-on: https://chromium-review.googlesource.com/c/1383031
Reviewed-by: Camille Lamy <clamy@chromium.org>
Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Michael van Ouwerkerk <mvanouwerkerk@chromium.org>
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623631}
27 files changed