Work around Firefox's excessive style security paranoia

Apparently it's possible for a stylesheet's .cssRules to be accessible,
but for the individual items' .cssText not to be, in which case Firefox
throws a nice security exception that kills the entire editor.

Wrap all inspector of cssRules in a try-catch, and fall back to cloning
the ownerNode if anything weird happens.

Change-Id: I0187ad308e395e92aea587c00fd8d8eb0280822e
Merge "Work around Firefox's excessive style security paranoia"