Bug 1542263 - Measure the styleset memory usage from the document, not the pres shell.

dom/base/Document.cpp

@@ -11155,6 +11155,8 @@ void Document::DocAddSizeOfExcludingThis(nsWindowSizes& aWindowSizes) const {
     mPresShell->AddSizeOfIncludingThis(aWindowSizes);
   }
 
+  mStyleSet->AddSizeOfIncludingThis(aWindowSizes);
+
   aWindowSizes.mDOMOtherSize += mLangGroupFontPrefs.SizeOfExcludingThis(
       aWindowSizes.mState.mMallocSizeOf);
 

layout/base/PresShell.cpp

@@ -10559,8 +10559,6 @@ void PresShell::AddSizeOfIncludingThis(nsWindowSizes& aSizes) const {
       mPendingScrollAnchorSelection.ShallowSizeOfExcludingThis(mallocSizeOf) +
       mPendingScrollAnchorAdjustment.ShallowSizeOfExcludingThis(mallocSizeOf);
 
-  StyleSet()->AddSizeOfIncludingThis(aSizes);
-
   aSizes.mLayoutTextRunsSize += SizeOfTextRuns(mallocSizeOf);
 
   aSizes.mLayoutPresContextSize +=