Add ability to wire up sharelist in glcontext creation

src/gpu/gl/egl/SkCreatePlatformGLContext_egl.cpp

Index: src/gpu/gl/egl/SkCreatePlatformGLContext_egl.cpp
diff --git a/src/gpu/gl/egl/SkCreatePlatformGLContext_egl.cpp b/src/gpu/gl/egl/SkCreatePlatformGLContext_egl.cpp
index 3b2488dfc6ecba364f185d383c3eec4ff70708a4..4ecdec7e9b65555bd40d7e761c8e9b076a578fd7 100644
--- a/src/gpu/gl/egl/SkCreatePlatformGLContext_egl.cpp
+++ b/src/gpu/gl/egl/SkCreatePlatformGLContext_egl.cpp
@@ -317,7 +317,8 @@ void SkEGLFenceSync::deleteFence(SkPlatformGpuFence platformFence) const {
 
 } // anonymous namespace
 
-SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI) {
+SkGLContext* SkCreatePlatformGLContext(GrGLStandard forcedGpuAPI, SkGLContext* shareContext) {
+    SkASSERT(!shareContext);

[bsalomon, 2016/01/20 15:26:07]

Why these asserts rather than just return nullptr?

[joshualitt, 2016/01/20 15:45:20]

I can see it both ways.  It make it easier to spot the failure because the
program will halt at the offending line, instead of just getting a null context
and not knowing why.  Also, I didn't want to clutter up all of these functions
with an additional check.

Of course, this means it won't work in release builds, which is unfortunate.

If you have a preference for returning null, then I can go that route.

     EGLGLContext* ctx = new EGLGLContext(forcedGpuAPI);
     if (!ctx->isValid()) {
         delete ctx;