Revert of Remove obsolete PauseableHttpProtocolHandler class.

ios/net/crn_http_protocol_handler.mm

Index: ios/net/crn_http_protocol_handler.mm
diff --git a/ios/net/crn_http_protocol_handler.mm b/ios/net/crn_http_protocol_handler.mm
index d2855b4e2aebe73594100d0c81cdb470557ab22d..bb9ac3b279565d8253a5c71e5f389356d34f57f7 100644
--- a/ios/net/crn_http_protocol_handler.mm
+++ b/ios/net/crn_http_protocol_handler.mm
@@ -1030,3 +1030,77 @@
 }
 
 @end
+
+#pragma mark -
+#pragma mark PauseableHttpProtocolHandler
+
+// The HttpProtocolHandler is called by the iOS system to handle the
+// NSURLRequest. This HttpProtocolHandler conforms to the observed semantics of
+// NSURLProtocol when used with NSURLSession on iOS 8 - i.e., |-startLoading|
+// means "start or resume request" and |-stopLoading| means "pause request".
+// Since there is no way to actually pause a request in the network stack, this
+// is implemented using a subclass of CRNHTTPProtocolHandlerProxy that knows how
+// to defer callbacks.
+//
+// Note that this class conforms to somewhat complex threading rules:
+// 1) |initWithRequest:cachedResponse:client:| and |dealloc| can be called on
+//    any thread.
+// 2) |startLoading| and |stopLoading| are always called on the client thread.
+// 3) |stopLoading| is called before |dealloc| is called.
+//
+// The main wrinkle is that |dealloc|, which may be called on any thread, needs
+// to clean up a running network request. To do this, |dealloc| needs to run
+// |cancelRequest|, which needs to be run on the client thread. Since it is
+// guaranteed that |startLoading| is called before |dealloc| is called, the
+// |startLoading| method stores a pointer to the client thread, then |dealloc|
+// asks that client thread to perform the |cancelRequest| selector via
+// |scheduleCancelRequest|.
+//
+// Some of the above logic is implemented in the parent class
+// (CRNHTTPProtocolHandler) because it is convenient.
+@implementation CRNPauseableHTTPProtocolHandler {
+  BOOL _started;
+  dispatch_queue_t _queue;
+}
+
+#pragma mark NSURLProtocol methods
+
+- (void)dealloc {
+  [self scheduleCancelRequest];
+}
+
+#pragma mark NSURLProtocol overrides.
+
+- (void)startLoading {
+  if (_started) {
+    [[self getProtocolHandlerProxy] resume];
+    return;
+  }
+
+  _started = YES;
+  [super startLoading];
+}
+
+- (void)stopLoading {
+  [[self getProtocolHandlerProxy] pause];
+}
+
+// This method has unusual concurrency properties. It can be called on any
+// thread, but it must be called from |-dealloc|, which guarantees that no other
+// method of this object is running concurrently (since |-dealloc| is only
+// called when the last reference to the object drops).
+//
+// This method takes a reference to _core to ensure that _core lives long enough
+// to have the request cleanly cancelled.
+- (void)scheduleCancelRequest {
+  DeferredCancellation* cancellation =
+      [[DeferredCancellation alloc] initWithCore:[self getCore]];
+  NSArray* modes = @[ [[NSRunLoop currentRunLoop] currentMode] ];
+  [cancellation performSelector:@selector(cancel)
+                       onThread:[self getClientThread]
+                     withObject:nil
+                  waitUntilDone:NO
+                          modes:modes];
+}
+
+@end