Refactoring BufferedDataSource to work with WebURLLoader instead of a MediaResourceLoaderBridge.

Attempt #2 after fixing some race conditions between Initialize() and Abort() (see r68363).

One thing to notice is that both buffered_data_source_unittest and simple_data_source_unittest need to have a way to inject a MockWebURLLoader into the BufferedResourceLoader and the SimpleDataSource.  In order to make sure a new one is not created during a Start(), I introduced the function SetURLLoaderForTest and keep_test_loader flag.

Patch by annacc@chromium.org:
http://codereview.chromium.org/3863002/

BUG=16751
TEST=src/xcodebuild/Debug/test_shell_tests --gtest_filter=Buffered*
src/xcodebuild/Debug/test_shell_tests --gtest_filter=Simple*
src/webkit/tools/layout_tests/run_webkit_tests.sh --debug media
webkit/tools/layout_tests/run_webkit_tests.sh --debug http/tests/media

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@68378 0039d316-1c4b-4281-b951-d872f2087c98