add more tests for discardable caches

src/core/SkScaledImageCache.cpp

Index: src/core/SkScaledImageCache.cpp
diff --git a/src/core/SkScaledImageCache.cpp b/src/core/SkScaledImageCache.cpp
index 5b166887282ebaf7889e779519593e5fa6753ae2..b3956f4a8dbd43d225dccd159eab687e572bd63e 100644
--- a/src/core/SkScaledImageCache.cpp
+++ b/src/core/SkScaledImageCache.cpp
@@ -209,6 +209,7 @@ private:
     SkDiscardableMemory* fDM;
     size_t               fRB;
     bool                 fFirstTime;
+    bool                 fIsLocked;
 
     typedef SkPixelRef INHERITED;
 };
@@ -224,6 +225,7 @@ SkOneShotDiscardablePixelRef::SkOneShotDiscardablePixelRef(const SkImageInfo& in
 
     SkASSERT(dm->data());
     fFirstTime = true;
+    fIsLocked = false;
 }
 
 SkOneShotDiscardablePixelRef::~SkOneShotDiscardablePixelRef() {
@@ -236,12 +238,18 @@ void* SkOneShotDiscardablePixelRef::onLockPixels(SkColorTable** ctable) {
         fFirstTime = false;
         return fDM->data();
     }
-    return fDM->lock() ? fDM->data() : NULL;
+
+    SkASSERT(!fIsLocked);
+    fIsLocked = fDM->lock();
+    return fIsLocked ? fDM->data() : NULL;
 }
 
 void SkOneShotDiscardablePixelRef::onUnlockPixels() {
     SkASSERT(!fFirstTime);
-    fDM->unlock();
+    if (fIsLocked) {
+        fIsLocked = false;
+        fDM->unlock();
+    }
 }
 
 size_t SkOneShotDiscardablePixelRef::getAllocatedSizeInBytes() const {