Add ability to wire up sharelist in glcontext creation

src/gpu/gl/iOS/SkCreatePlatformGLContext_iOS.mm

Index: src/gpu/gl/iOS/SkCreatePlatformGLContext_iOS.mm
diff --git a/src/gpu/gl/iOS/SkCreatePlatformGLContext_iOS.mm b/src/gpu/gl/iOS/SkCreatePlatformGLContext_iOS.mm
index b90948af442167944f223a6ef748dc8ed28c952e..5be351f52e8414494954bab11cbba3edcd7f33ee 100644
--- a/src/gpu/gl/iOS/SkCreatePlatformGLContext_iOS.mm
+++ b/src/gpu/gl/iOS/SkCreatePlatformGLContext_iOS.mm
@@ -89,7 +89,11 @@ GrGLFuncPtr IOSGLContext::onPlatformGetProcAddress(const char* procName) const {
 
 } // anonymous namespace
 
-SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI) {
+SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI, SkGLContext* shareContext) {
+    SkASSERT(!shareContext);
+    if (shareContext) {
+        return NULL;
+    }
     if (kGL_GrGLStandard == forcedGpuAPI) {
         return NULL;
     }