Fix a typo in webview_custom_bindings.js

BUG=140315
Test=None, this bindings is not exposed yet.

Review URL: https://codereview.chromium.org/189923003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255840 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255840