Revert of Remove support for Alt-Svc/Alternate Protocol Probability

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 17 matching lines @@
 // effect before |install| is called.
 + (void)setQuicEnabled:(BOOL)quicEnabled;
 
 // Sets whether SDCH should be supported by CrNet. This method only has any
 // effect before |install| is called. The |filename| argument is used to specify
 // which file should be used for SDCH persistence metadata. If |filename| is
 // nil, persistence is not enabled. The default is for SDCH to be disabled.
 + (void)setSDCHEnabled:(BOOL)sdchEnabled
          withPrefStore:(NSString*)filename;
 
+// Set the alternate protocol threshold. Servers announce alternate protocols
+// with a probability value; any alternate protocol whose probability value is
+// greater than this value will be used, so |alternateProtocolThreshold| == 0
+// implies any announced alternate protocol will be used, and
+// |alternateProtocolThreshold| == 1 implies no alternate protocol will ever be
+// used. Note that individual alternate protocols must also be individually
+// enabled to be considered; currently the only alternate protocol is QUIC (SPDY
+// is not controlled by this mechanism).
+//
+// For example, imagine your service has two frontends a.service.com and
+// b.service.com, and you would like to divide your users into three classes:
+//   Users who use QUIC for both a and b
+//   Users who use QUIC for a but not b
+//   Users who use QUIC for neither a nor b
+// You can achieve that effect with:
+//   a.service.com advertises QUIC with p=0.67
+//   b.service.com advertises QUIC with p=0.33
+//   alternateProtocolThreshold set to a uniform random number in [0,1]
+// Now equal proportions of users will fall into the three experimental groups.
+//
+// The default for this value is 1.0, i.e. all alternate protocols disabled.
++ (void)setAlternateProtocolThreshold:(double)alternateProtocolThreshold;
+
 // |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;
 
 // 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|.
@@ skipping 59 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