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- Don't alias namespaces in header files as per
  https://google.github.io/styleguide/cppguide.html#Namespaces
- A few iwyu cleanups.
- Use AddAllocated to pass an element's memory to a repeated proto
  field.
- Move a pair of funcs in BrowserReportGenerator into an unnamed
  namespace.
- Avoid copies when iterating over plugins in
  BrowserReportGenerator::OnPluginsReady.

BUG=none

Change-Id: Ib47925de22d6a0872f40e068a246183888736046
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2091493
Commit-Queue: Greg Thompson <grt@chromium.org>
Reviewed-by: Owen Min <zmin@chromium.org>
Auto-Submit: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747888}