Add WebKit brace style configuration option.

Summary:
Add brace style `BS_WebKit` as described on https://www.webkit.org/coding/coding-style.html:

* Function definitions: place each brace on its own line.
* Other braces: place the open brace on the line preceding the code block; place the close brace on its own line.

Set brace style used in `getWebKitStyle()` to the newly added `BS_WebKit`.

Reviewers: djasper, klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D11837

llvm-svn: 244446