IFrames blocked via CSP generate 'load' events.

The platform_apps/IFrames test fails after [1], as it relies on the data
leakage that CL prevents. Note that the IFrame itself would generate a
test failure if it indeed loaded. Checking the 'load' event was redundant
and is now incorrect.

[1]: https://codereview.chromium.org/143323012/

Review URL: https://codereview.chromium.org/145643003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246748 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246748