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
In next part, we'll use ServoStyleSet to host XBL stylesheets. The raw set
is not available until the resources are loaded, so it's easier to tell that
by checking whether the pointer is nullptr or not.
MozReview-Commit-ID: Hqydd55FPO9
