Bug 1435939: Make media feature changes always async. r=bz

Much in the spirit of bug 1434474.

We right now call MediaFeatureChanges sync or async pretty randomly. This has
caused bugs in the past like bug 1413143.

Unify media feature changes, and only post them async, and flush them from
FlushPendingNotifications.

This also fixes a pre-existing problem where style wasn't flushed correctly from
getComputedStyle when there were pending media feature values.

MozReview-Commit-ID: H9S1M8fk5H4

docshell/base/nsDocShell.cpp

@@ -25,6 +25,7 @@
 #include "mozilla/HTMLEditor.h"
 #include "mozilla/LoadInfo.h"
 #include "mozilla/Logging.h"
+#include "mozilla/MediaFeatureChange.h"
 #include "mozilla/Preferences.h"
 #include "mozilla/ResultExtensions.h"
 #include "mozilla/Services.h"
@@ -4179,7 +4180,8 @@ nsDocShell::SetDeviceSizeIsPageSize(bool aValue)
     RefPtr<nsPresContext> presContext;
     GetPresContext(getter_AddRefs(presContext));
     if (presContext) {
-      presContext->MediaFeatureValuesChanged(nsRestyleHint(0));
+      presContext->MediaFeatureValuesChanged({
+        MediaFeatureChangeReason::DeviceSizeIsPageSizeChange });
     }
   }
   return NS_OK;
@@ -14423,7 +14425,8 @@ nsDocShell::SetDisplayMode(uint32_t aDisplayMode)
 
     RefPtr<nsPresContext> presContext;
     if (NS_SUCCEEDED(GetPresContext(getter_AddRefs(presContext)))) {
-      presContext->MediaFeatureValuesChangedAllDocuments(nsRestyleHint(0));
+      presContext->MediaFeatureValuesChangedAllDocuments({
+        MediaFeatureChangeReason::DisplayModeChange });
     }
   }