Add ability to wire up sharelist in glcontext creation

src/gpu/gl/mac/SkCreatePlatformGLContext_mac.cpp

Index: src/gpu/gl/mac/SkCreatePlatformGLContext_mac.cpp
diff --git a/src/gpu/gl/mac/SkCreatePlatformGLContext_mac.cpp b/src/gpu/gl/mac/SkCreatePlatformGLContext_mac.cpp
index 41585c8632ece4da3b1cfc5ccdcaa0e69853d93c..d1826a49e3eab379fbb3d48e7a860a27d1ff4fdd 100644
--- a/src/gpu/gl/mac/SkCreatePlatformGLContext_mac.cpp
+++ b/src/gpu/gl/mac/SkCreatePlatformGLContext_mac.cpp
@@ -109,7 +109,12 @@ GrGLFuncPtr MacGLContext::onPlatformGetProcAddress(const char* procName) const {
 
 } // anonymous namespace
 
-SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI) {
+SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI, SkGLContext* shareContext) {
+    SkASSERT(!shareContext);
+    if (shareContext) {
+        return nullptr;
+    }
+
     if (kGLES_GrGLStandard == forcedGpuAPI) {
         return nullptr;
     }