Bug 1298588 part 4, gecko piece. Recreate the default computed values for a document as needed. r=bholley

layout/base/ServoRestyleManager.cpp

@@ -87,6 +87,18 @@ ServoRestyleManager::RebuildAllStyleData(nsChangeHint aExtraHint,
                                          nsRestyleHint aRestyleHint)
 {
   NS_WARNING("stylo: ServoRestyleManager::RebuildAllStyleData not implemented");
+  // That said, we do know that rebuilding all style data in Gecko would get rid
+  // of the old ruletree, and hence of the cached-on-the-root default computed
+  // styles.  So we know we need to clear them here.  I think this is the only
+  // way they could get cleared, in fact, though not _all_ calls that come
+  // through here may need to clear them in practice.
+  //
+  // We probably need to do some actual restyling here too, though.  And figure
+  // out whether it actually matters that we may be recomputing the default
+  // styles in too many cases.  For one thing, we do a bunch of eager work here,
+  // whereas we should really just set a bit that says to recompute the default
+  // computed styles before the next time we restyle anything!
+  StyleSet()->RecomputeDefaultComputedStyles();
 }
 
 void

layout/style/ServoBindingList.h

@@ -51,6 +51,9 @@ SERVO_BINDING_FUNC(Servo_StyleSheet_HasRules, bool,
 SERVO_BINDING_FUNC(Servo_StyleSheet_GetRules, ServoCssRulesStrong,
                    RawServoStyleSheetBorrowed sheet)
 SERVO_BINDING_FUNC(Servo_StyleSet_Init, RawServoStyleSetOwned, RawGeckoPresContextBorrowed pres_context)
+SERVO_BINDING_FUNC(Servo_StyleSet_RecomputeDefaultStyles, void,
+                   RawServoStyleSetBorrowed set,
+                   RawGeckoPresContextBorrowed pres_context)
 SERVO_BINDING_FUNC(Servo_StyleSet_Drop, void, RawServoStyleSetOwned set)
 SERVO_BINDING_FUNC(Servo_StyleSet_AppendStyleSheet, void,
                    RawServoStyleSetBorrowed set, RawServoStyleSheetBorrowed sheet, bool flush)

layout/style/ServoStyleSet.cpp

@@ -550,3 +550,9 @@ ServoStyleSet::AssertTreeIsClean()
   }
 }
 #endif
+
+void
+ServoStyleSet::RecomputeDefaultComputedStyles()
+{
+  Servo_StyleSet_RecomputeDefaultStyles(mRawSet.get(), mPresContext);
+}

layout/style/ServoStyleSet.h

@@ -162,6 +162,12 @@ public:
   void AssertTreeIsClean() {}
 #endif
 
+  /**
+   * Recompute our default computed styles.  This will eagerly create a new set
+   * of default computed style structs.
+   */
+  void RecomputeDefaultComputedStyles();
+
 private:
   already_AddRefed<nsStyleContext> GetContext(already_AddRefed<ServoComputedValues>,
                                               nsStyleContext* aParentContext,