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Remove unneeded include from app_window_contents.cc
Move app_window_contents.{cc,h} to extensions/browser/app_window
Change the namespace from apps to extensions.

BUG=403726
TBR=derat@chromium.org for include&namespace fix

Review URL: https://codereview.chromium.org/527693002

Cr-Commit-Position: refs/heads/master@{#293094}