This patch basically does:
* remove StyleSetHandle and its corresponding files
* revisit #includes of related header files and change correspondingly
* change nsIPresShell::mStyleSet to be UniquePtr<ServoStyleSet>
* change the creating path of ServoStyleSet to pass UniquePtr
* change other mentions of StyleSetHandle to ServoStyleSet*
* remove AsServo() calls on ServoStyleSet
Some unfortunate bits:
* some methods of (Servo)StyleSet only accepts ServoStyleSheet while
many places call into the methods with StyleSheet, so there are many
->AsServo() added to sheets
MozReview-Commit-ID: K4zYnuhOurA
ea8b8b1e74