[Cronet] Add CrNet.setUserAgent method on iOS.

ios/crnet/CrNet.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import <Foundation/Foundation.h>
 
 // A block, that takes a request, and returns YES if the request should
 // be handled.
 typedef BOOL(^RequestFilterBlock)(NSURLRequest *request);
 
@@ skipping 24 matching lines @@
 // nil, persistence is not enabled. The default is for SDCH to be disabled.
 + (void)setSDCHEnabled:(BOOL)sdchEnabled
          withPrefStore:(NSString *)filename;
 
 // |userAgent| is expected to be of the form Product/Version.
 // Example: Foo/3.0.0.0
 //
 // 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
 // either before or after |install|.
 + (void)setRequestFilterBlock:(RequestFilterBlock)block;
 
 // Installs CrNet. Once installed, CrNet intercepts and handles all
 // NSURLConnection and NSURLRequests issued by the app, including UIWebView page
 // loads. It is recommended to call this method on the application main thread.
 // If the method is called on any thread other than the main one, the method
@@ skipping 57 matching lines @@
 
 // Returns the full user-agent that the stack uses.
 // This is the exact string servers will see.
 + (NSString *)userAgent;
 
 // Clears CrNet's http cache. The supplied callback, if not nil, is run on an
 // unspecified thread.
 + (void)clearCacheWithCompletionCallback:(ClearCacheCallback)completionBlock;
 
 @end