[ios] Fixes the repo URL for ios/third_party/webkit/src.

BUG=None

Change-Id: I07de8b010065ea27a5f36bdd5f71d12a5bde7ab5
Reviewed-on: https://chromium-review.googlesource.com/c/1457003
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629837}

DEPS

@@ -449,7 +449,7 @@ deps = {
   },
 
   'src/ios/third_party/webkit/src': {
-      'url': 'https://chromium.googlesource.com/external/Webkit',
+      'url': Var('chromium_git') + '/external/github.com/WebKit/webkit.git',
       'condition': 'checkout_ios and checkout_ios_webkit'
   },