[ios] Adds /usr/bin to the path when running build_webkit.

The webkit build invokes the "copypng" command, which is in /usr/bin.

BUG=None

Change-Id: I3a343f5620531665bc6d0f3f7a73c7e84c290912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2220881Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773295}

ios/third_party/webkit/build_webkit.py

@@ -45,7 +45,10 @@ def main():
     command.extend(['-jobs', opts.j])
   command.extend(extra_args)
 
-  env = {'WEBKIT_OUTPUTDIR': output_dir}
+  env = {
+    'WEBKIT_OUTPUTDIR': output_dir,
+    'PATH': '/usr/bin',  # Needed for /usr/bin/copypng
+  }
   cwd = os.path.dirname(os.path.realpath(__file__))
 
   if opts.clean: