Bug 1390691 - Fix up Servo_StyleSet_MediumFeaturesChanged for Linux 32-bit ABI. r=manishearth

Bindgen bitfield enums don't work as return values with the Linux 32-bit ABI at
the moment because they wrap the value in a struct.

This causes the Rust side to believe the caller will pass along space for the
struct return value, while C++ believes it's just an integer value.

MozReview-Commit-ID: LY6z7lEKgOp

--HG--
extra : rebase_source : deb9739bd100e2162e7c93d6d45d7029d7793355

layout/style/ServoBindingList.h

@@ -63,7 +63,10 @@ SERVO_BINDING_FUNC(Servo_StyleSet_Clear, void,
                    RawServoStyleSetBorrowed set)
 SERVO_BINDING_FUNC(Servo_StyleSet_RebuildCachedData, void,
                    RawServoStyleSetBorrowed set)
-SERVO_BINDING_FUNC(Servo_StyleSet_MediumFeaturesChanged, mozilla::OriginFlags,
+// We'd like to return `OriginFlags` here, but bindgen bitfield enums don't
+// work as return values with the Linux 32-bit ABI at the moment because
+// they wrap the value in a struct.
+SERVO_BINDING_FUNC(Servo_StyleSet_MediumFeaturesChanged, uint8_t,
                    RawServoStyleSetBorrowed set, bool* viewport_units_used)
 SERVO_BINDING_FUNC(Servo_StyleSet_Drop, void, RawServoStyleSetOwned set)
 SERVO_BINDING_FUNC(Servo_StyleSet_CompatModeChanged, void,

layout/style/ServoStyleSet.cpp

@@ -167,8 +167,8 @@ nsRestyleHint
 ServoStyleSet::MediumFeaturesChanged(bool aViewportChanged)
 {
   bool viewportUnitsUsed = false;
-  const OriginFlags rulesChanged =
-    Servo_StyleSet_MediumFeaturesChanged(mRawSet.get(), &viewportUnitsUsed);
+  const OriginFlags rulesChanged = static_cast<OriginFlags>(
+    Servo_StyleSet_MediumFeaturesChanged(mRawSet.get(), &viewportUnitsUsed));
 
   if (rulesChanged != OriginFlags(0)) {
     MarkOriginsDirty(rulesChanged);