Prepare SVGLengthContext for oilpan

SVGLengthContext is made STACK_ALLOCATED, as it is currently never used
on a heap.

BUG=357163

Review URL: https://codereview.chromium.org/339793004

git-svn-id: svn://svn.chromium.org/blink/trunk@176268 bbb929c8-8fbe-4397-9dbb-9b2b20218538

third_party/WebKit/Source/core/svg/SVGLengthContext.h

@@ -50,6 +50,7 @@ enum SVGLengthMode {
 };
 
 class SVGLengthContext {
+    STACK_ALLOCATED();
 public:
     explicit SVGLengthContext(const SVGElement*);
 
@@ -80,7 +81,7 @@ private:
     float convertValueFromUserUnitsToEXS(float value, ExceptionState&) const;
     float convertValueFromEXSToUserUnits(float value, ExceptionState&) const;
 
-    const SVGElement* m_context;
+    RawPtrWillBeMember<const SVGElement> m_context;
     FloatRect m_overridenViewport;
 };