Fix missing StackFrameDepth constructor

TBR=haraken@chromium.org,oilpan-reviews@chromium.org
BUG=645253

Review-Url: https://codereview.chromium.org/2322123003
Cr-Commit-Position: refs/heads/master@{#417525}

third_party/WebKit/Source/platform/heap/StackFrameDepth.h

@@ -22,6 +22,7 @@ namespace blink {
 class PLATFORM_EXPORT StackFrameDepth final {
     DISALLOW_NEW();
 public:
+    StackFrameDepth(): m_stackFrameLimit(kMinimumStackLimit) {}
     bool isSafeToRecurse()
     {
         // Asssume that the stack grows towards lower addresses, which