Bug 935721, part 3 - Use RAII to set mScanInProgress, hoist out MarkRoots and ScanRoots. r=smaug

With ICC, mScanInProgress gets set and cleared a bunch of times so add an RAII class to turn
it on when we're doing stuff and clear it when we're not.

With that in place, we can easily move MarkRoots and ScanRoots out of BeginCollection
in preparation for making them separate phases in ICC.

xpcom/base/nsCycleCollector.cpp

@@ -98,12 +98,13 @@
 
 #include "base/process_util.h"
 
+#include "mozilla/AutoRestore.h"
+#include "mozilla/CycleCollectedJSRuntime.h"
 /* This must occur *after* base/process_util.h to avoid typedefs conflicts. */
 #include "mozilla/MemoryReporting.h"
 #include "mozilla/Util.h"
 #include "mozilla/LinkedList.h"
 
-#include "mozilla/CycleCollectedJSRuntime.h"
 #include "nsCycleCollectionParticipant.h"
 #include "nsCycleCollectionNoteRootCallback.h"
 #include "nsDeque.h"
@@ -2150,6 +2151,11 @@ nsCycleCollector::ForgetSkippable(bool aRemoveChildlessNodes,
 MOZ_NEVER_INLINE void
 nsCycleCollector::MarkRoots()
 {
+    TimeLog timeLog;
+    AutoRestore<bool> ar(mScanInProgress);
+    MOZ_ASSERT(!mScanInProgress);
+    mScanInProgress = true;
+
     mGraph.mRootCount = mBuilder->Count();
 
     // read the PtrInfo out of the graph that we are building
@@ -2172,6 +2178,7 @@ nsCycleCollector::MarkRoots()
     }
 
     mBuilder = nullptr;
+    timeLog.Checkpoint("MarkRoots()");
 }
 
 
@@ -2294,6 +2301,10 @@ nsCycleCollector::ScanWeakMaps()
 void
 nsCycleCollector::ScanRoots()
 {
+    TimeLog timeLog;
+    AutoRestore<bool> ar(mScanInProgress);
+    MOZ_ASSERT(!mScanInProgress);
+    mScanInProgress = true;
     mWhiteNodeCount = 0;
 
     // On the assumption that most nodes will be black, it's
@@ -2336,6 +2347,7 @@ nsCycleCollector::ScanRoots()
         mListener->End();
         mListener = nullptr;
     }
+    timeLog.Checkpoint("ScanRoots()");
 }
 
 
@@ -2714,6 +2726,8 @@ nsCycleCollector::Collect(ccType aCCType,
 
     PrepareForCollection(aResults, aWhiteNodes);
     BeginCollection(aCCType, aManualListener);
+    MarkRoots();
+    ScanRoots();
     bool collectedAny = CollectWhite();
     CleanupAfterCollection();
     return collectedAny;
@@ -2804,18 +2818,11 @@ nsCycleCollector::BeginCollection(ccType aCCType,
         timeLog.Checkpoint("mJSRuntime->BeginCycleCollection()");
     }
 
+    AutoRestore<bool> ar(mScanInProgress);
+    MOZ_ASSERT(!mScanInProgress);
     mScanInProgress = true;
     mPurpleBuf.SelectPointers(*mBuilder);
     timeLog.Checkpoint("SelectPointers()");
-
-    // The main Bacon & Rajan collection algorithm.
-    MarkRoots();
-    timeLog.Checkpoint("MarkRoots()");
-
-    ScanRoots();
-    timeLog.Checkpoint("ScanRoots()");
-
-    mScanInProgress = false;
 }
 
 uint32_t