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This is the first step to moving Web Bluetooth over to mojo. This patch
does the following:

- Creates a new WebBluetoothError mojo enum in a new web_bluetooth.mojom.
  This enum should be accessible from both content and blink.
- Modifies WebKit/public/BUILD.gn and WebKit/public/blink.gyp to
  generate the headers for the new mojom interface.
- Modifies WebKit/public/BUILD.gn, WebKit/Source/BUILD.gn,
  WebKit/public/blink_headers.gyp, and WebKit/Source/config.gyp to
  include files in gen/third_party/WebKit
- Modifies content/browser/DEPS so that the new headers can be accessed
  from content/browser.

Follow up patch with Web Bluetooth Service: http://crrev.com/1775953004

BUG=508771

Review URL: https://codereview.chromium.org/1784613002

Cr-Commit-Position: refs/heads/master@{#381525}