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This creates a new BUILD.gn file for everything in //aw/browser and
removes unused dependencies.

Test: ninja -C out/aw system_webview_apk; gn check out/aw android_webview/browser
Bug: 934152
Change-Id: I2f94e67d57c3a48687c44ddb475f529e67dc7389
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824018Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Robbie McElrath <rmcelrath@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699928}