Pin src/ios_internal ref in src/DEPS

To allow the autoroller to work, pin the reference to
src/ios_internal in src/DEPS. The hash used was found
using "git ls-remote $IOS_INTERNAL refs/heads/master".

Bug: 803846,skia:7552
Change-Id: I418a7cfa9db19729f19658ac16baf3e224638c86
Reviewed-on: https://chromium-review.googlesource.com/897950Reviewed-by: Eric Boren <borenet@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534062}

DEPS

@@ -261,8 +261,7 @@ deps = {
   },
 
   'src/ios_internal': {
-      # TODO(sdefresne): Leave "floating" until auto-roller is working.
-      'url': 'https://chrome-internal.googlesource.com/chrome/ios_internal.git' + '@' + 'refs/heads/master',
+      'url': 'https://chrome-internal.googlesource.com/chrome/ios_internal.git' + '@' + '671969ff8c01a14ef454a2ee9e0be1c020475990',
       'condition': 'checkout_ios and checkout_google_internal',
   },