[gn check] Enable //chrome/test:browser_tests

This involves the following:

- Eliminating many unneeded includes that cause unnecessary "gn check"
  problems
- Adding "nogncheck" to some conditionally-included headers in
  //content. This was a pre-existing issue (the relevant targets are
  conditionally depended on in GN, which doesn't take into account
  conditionality when analyzing includes); it's unclear why GN check
  started complaining about it with the changes in this CL, but the fix
  is correct in any case given the problem.
- Allow //chrome/test:browser_tests to depend on //pdf:pdf_internal.
  This is due to a real dependence of
  //chrome/browser/chrome_find_request_manager_browsertest.cc on
  //pdf/document_loader_impl.h for a constant defined in that header
  file.

Bug: 1158929
Change-Id: I87a082a1312306b918cfb730695cd3fada617ae3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2905653


Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#886224}