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This CL introduces no behavior changes.

WebAppProvider class must not depend on extensions,
only forward declarations are visible.

All extensions-specific code and includes have been isolated in
chrome/browser/web_applications/extensions/bookmark_app_provider.cc
file.

In long term, the whole chrome/browser/web_applications/extensions/
code directory must be deleted (see README.md in that folder)

This is a preparation CL to delete web_applications_on_extensions
GN source set.

TBR=ccameron@chromium.org

Bug: 1065748
Change-Id: I7fa1306694e80dbda8aab7159a075919ec2c42b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517233Reviewed-by: Alexey Baskakov <loyso@chromium.org>
Reviewed-by: Daniel Murphy <dmurph@chromium.org>
Reviewed-by: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Alexey Baskakov <loyso@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824327}