Add CRWWebViewScrollViewProxy.dragging which will be used in

ios/web/web_state/ui/crw_web_view_scroll_view_proxy_unittest.mm

Index: ios/web/web_state/ui/crw_web_view_scroll_view_proxy_unittest.mm
diff --git a/ios/web/web_state/ui/crw_web_view_scroll_view_proxy_unittest.mm b/ios/web/web_state/ui/crw_web_view_scroll_view_proxy_unittest.mm
index 2f3581b8e2a80d127fe238e4744a472cce1c19ef..f1c17ddd0eba61a293c0936db6f4d0e1fd21da3c 100644
--- a/ios/web/web_state/ui/crw_web_view_scroll_view_proxy_unittest.mm
+++ b/ios/web/web_state/ui/crw_web_view_scroll_view_proxy_unittest.mm
@@ -101,6 +101,12 @@ TEST_F(CRWWebViewScrollViewProxyTest, testScrollViewPresent) {
   [[[mockScrollView_ stub] andReturnValue:[NSValue valueWithCGRect:frame]]
       frame];
   EXPECT_TRUE(CGRectEqualToRect(frame, [webViewScrollViewProxy_ frame]));
+
+  [[[mockScrollView_ expect] andReturnValue:@YES] isDragging];
+  EXPECT_TRUE([webViewScrollViewProxy_ isDragging]);
+
+  [[[mockScrollView_ expect] andReturnValue:@NO] isDragging];
+  EXPECT_FALSE([webViewScrollViewProxy_ isDragging]);
 }
 
 // Tests that CRWWebViewScrollViewProxy returns the correct property values when
@@ -117,6 +123,7 @@ TEST_F(CRWWebViewScrollViewProxyTest, testScrollViewAbsent) {
   EXPECT_TRUE(
       CGSizeEqualToSize(CGSizeZero, [webViewScrollViewProxy_ contentSize]));
   EXPECT_TRUE(CGRectEqualToRect(CGRectZero, [webViewScrollViewProxy_ frame]));
+  EXPECT_FALSE([webViewScrollViewProxy_ isDragging]);
 
   // Make sure setting the properties is fine too.
   // Arbitrary point.