[Documentation] Update Objective C style guide

- Correct the URL of the public Google ObjC style guide.
- Correct the guide to no longer assert that Google ObjC style "inherits"
  from C++ style.

TBR: eugenebut@chromium.org
Bug: 
Change-Id: I67c42c648536d8c0d1fbcd3cfef98424c811bdb1
Reviewed-on: https://chromium-review.googlesource.com/725327Reviewed-by: Mark Cogan <marq@chromium.org>
Commit-Queue: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#509738}

styleguide/objective-c/objective-c.md

@@ -3,7 +3,7 @@
 _For other languages, please see the [Chromium style guides](https://chromium.googlesource.com/chromium/src/+/master/styleguide/styleguide.md)._
 
 Chromium follows the
-[Google Objective-C style guide](https://google.github.io/styleguide/objcguide.xml)
+[Google Objective-C style guide](https://github.com/google/styleguide/blob/gh-pages/objcguide.md)
 unless an exception is listed below.
 
 A checkout should give you
@@ -19,9 +19,8 @@ Objective-C and Objective-C++ code also has an 80 character line length.
 
 ## Chromium C++ style
 
-Just as [Google Objective-C style](https://google.github.io/styleguide/objcguide.xml)
-follows [Google C++ Style](https://google.github.io/styleguide/cppguide.html),
-Chromium Objective-C and Objective-C++ follows [Chromium C++ style](../c++/c++.md).
+Where appropriate, the [Chromium C++ style](../c++/c++.md) style guide applies
+to Chromium Objective-C and (especially) Objective-C++
 
 ## Code Formatting