Add Swift style guide directions

As there are no exceptions at this moment, it only points to Google's
style guide.

Bug: N/A
Change-Id: Ie5102b3152e38394d0482203dfeb57bfc94c648e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2555461Reviewed-by: Mike Pinkerton <pinkerton@chromium.org>
Commit-Queue: Javier Flores <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834813}

styleguide/styleguide.md

@@ -5,6 +5,7 @@
   * [Chromium C++ style guide](c++/c++.md)
     * See also: [C++ Dos and Don'ts](c++/c++-dos-and-donts.md) for Chromium best-practices.
   * [Chromium Objective-C style guide](objective-c/objective-c.md)
+  * [Chromium Swift style guide](swift/swift.md)
   * [Java style guide for Android](java/java.md)
   * [Chromium Python style guide](python/python.md)
   * [GN style guide](https://gn.googlesource.com/gn/+/master/docs/style_guide.md) for build files.

styleguide/swift/OWNERS

+javierrobles@chromium.org
+pinkerton@chromium.org

styleguide/swift/swift.md

+# Chromium Swift style guide
+
+_For other languages, please see the [Chromium style guides](https://chromium.googlesource.com/chromium/src/+/master/styleguide/styleguide.md)._
+
+Chromium follows the
+[Google Swift style guide](https://google.github.io/swift/)
+no exceptions at this point.
+
+[swift-format](https://github.com/apple/swift-format)
+can be used to automatically format Swift code.