Add ability to wire up sharelist in glcontext creation

src/gpu/gl/win/SkCreatePlatformGLContext_win.cpp

Index: src/gpu/gl/win/SkCreatePlatformGLContext_win.cpp
diff --git a/src/gpu/gl/win/SkCreatePlatformGLContext_win.cpp b/src/gpu/gl/win/SkCreatePlatformGLContext_win.cpp
index f795f525547daf0a9542b1ffef12e669af5121d3..6cc11439bf9153b82df924d8534c83c6eed960c0 100644
--- a/src/gpu/gl/win/SkCreatePlatformGLContext_win.cpp
+++ b/src/gpu/gl/win/SkCreatePlatformGLContext_win.cpp
@@ -187,7 +187,11 @@ GrGLFuncPtr WinGLContext::onPlatformGetProcAddress(const char* name) const {
 
 } // anonymous namespace
 
-SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI) {
+SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI, SkGLContext* shareContext) {
+    SkASSERT(!shareContext);
+    if (shareContext) {
+        return nullptr;
+    }
     WinGLContext* ctx = new WinGLContext(forcedGpuAPI);
     if (!ctx->isValid()) {
         delete ctx;