A mochitest browser test for image blocking.
We query the blocking status by reading imageBlockingStatus.
See nsImageLoadingContent.cpp for the logic of blocking image.
In this test we verified the following behavior:
1. image is loaded.
2. image is blocked.
3. mCurrentRequest doesn't have size yet, so it should be replaced.
4. mCurrentRequest already got size, the following request should be a
pendingRequest.
