Should this be in the drp component or the previews component?

Indeed an interesting question. One point of view is that the knowledge of drp headers should be in the drp component, so this particular update logic related to those headers should be in drp. I don't know if we later end up with some split of utilities (drp headers utilities vs. some previews state update utilities) as client side previews get more involved.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9ee542da746d166a094a705ed3fcd7304dfe61e0

commit 9ee542da746d166a094a705ed3fcd7304dfe61e0
Author: megjablon <megjablon@chromium.org>
Date: Mon Aug 07 23:54:26 2017

Factor DRP PreviewsState logic out of content

Move the Data Reduction Proxy header related logic for updating Previews
state out of content and into a RenderFrameObserver in the Data Reduction
Proxy component.

BUG= 737159 

Review-Url: https://codereview.chromium.org/2963913002
Cr-Commit-Position: refs/heads/master@{#492458}

[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/chrome/renderer/BUILD.gn
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/chrome/renderer/DEPS
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/chrome/renderer/chrome_content_renderer_client.cc
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/BUILD.gn
[add] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/data_reduction_proxy/content/renderer/BUILD.gn
[add] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/data_reduction_proxy/content/renderer/DEPS
[add] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/data_reduction_proxy/content/renderer/content_previews_render_frame_observer.cc
[add] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/data_reduction_proxy/content/renderer/content_previews_render_frame_observer.h
[add] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/data_reduction_proxy/content/renderer/content_previews_render_frame_observer_unittest.cc
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.cc
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/components/data_reduction_proxy/core/common/data_reduction_proxy_headers.h
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/content/public/renderer/render_frame.h
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/content/renderer/BUILD.gn
[delete] https://crrev.com/b79ebf26322a0a9f0af1ecf8bc844cb8a7b218a4/content/renderer/previews_state_helper.cc
[delete] https://crrev.com/b79ebf26322a0a9f0af1ecf8bc844cb8a7b218a4/content/renderer/previews_state_helper.h
[delete] https://crrev.com/b79ebf26322a0a9f0af1ecf8bc844cb8a7b218a4/content/renderer/previews_state_helper_unittest.cc
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/content/renderer/render_frame_impl.cc
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/content/renderer/render_frame_impl.h
[modify] https://crrev.com/9ee542da746d166a094a705ed3fcd7304dfe61e0/content/test/BUILD.gn