Push styles only to the main frame of a tab; let the content script pull styles into child frames, as before.
`chrome.tabs.sendMessage` can only broadcast to all frames in a tab until Chrome 41. The main frame always fires `onCommit` before any child frame, so only the main frame hears its own message from `onCommit`. It (and every child frame that's already fired `onCommit`) hears the `onCommit` message for every frame that follows, with the result that the main frame applies all the styles for every frame in the tab.
https://forum.userstyles.org/discussion/45062/stylish-for-chrome-1-3-0b1#Comment_95474
