Fix issue: when overriding keybinding preferences, should remove the one taken.

Also including the following things:
1. Fix a problem that introduced by my previous CL[0].
In the ExtensionKeybindingRegistry::RemoveExtensionKeybinding, we should break
only when we really find the specified command.
2. Test case for removal of keybinding preference should be platform-specific.


[0]: https://codereview.chromium.org/64273008/

BUG=302437
TEST=interactive_ui_tests --gtest_filter=CommandsApiTest.UpdateKeybindingPrefsTest
TEST=browser_test --gtest_filter=CommandServiceTest.RemoveKeybindingPrefsShouldBePlatformSpecific
TBR=finnur@chromium.org

Review URL: https://codereview.chromium.org/94723008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238587 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238587