[Cronet] Add CrNet.setUserAgent method on iOS.

ios/crnet/CrNet.h

Index: ios/crnet/CrNet.h
diff --git a/ios/crnet/CrNet.h b/ios/crnet/CrNet.h
index 0be9b7769e0e9db2f12176e64cadb09e68e807a1..43c8f08fbc9caa5f28d900369c67371b4110a184 100644
--- a/ios/crnet/CrNet.h
+++ b/ios/crnet/CrNet.h
@@ -42,6 +42,12 @@ __attribute__((visibility("default")))
 // This method only has any effect before |install| is called.
 + (void)setPartialUserAgent:(NSString *)userAgent;
 
+// |userAgent| is expected to be the complete user agent value sent to remote.
+// Example: Foo/3.0.0.0

[droger, 2016/08/31 08:52:23]

This is the same example as setPartialUserAgent above, and it's not clear from
the documentation what is the difference.

Could we improve the doc (at least the example)?

We could also mention that calling setPartialUserAgent undoes previous calls to
setUserAgent and vice-versa.

Or an alternative would be to have the function be:
+ (void)setUserAgent:(NSString*)userAgent partial:(bool)partial;

Then you can remove the other one, or keep it for backward compatibility (and in
this case the comment could be that it does the same thing as calling
setUserAgent with partial=false).

[mef, 2016/08/31 14:49:57]

Great suggestion! Done.

+//
+// This method only has any effect before |install| is called.
++ (void)setUserAgent:(NSString*)userAgent;
+
 // Set the block used to determine whether or not CrNet should handle the
 // request. If this is not set, CrNet will handle all requests.
 // Must not be called while requests are in progress. This method can be called