Support using OpenGL ES context on desktop

Support using OpenGL ES context on desktop for unix and Android platforms. This
is mainly useful in development.

Add --gpuAPI flag to gm, dm, bench, bench_pictures and render_pictures. The
possible parameters for the flag are "gl" and "gles".

R=bsalomon@google.com, mtklein@google.com, robertphillips@google.com

Author: kkinnunen@nvidia.com

Review URL: https://codereview.chromium.org/319043005
diff --git a/src/gpu/gl/SkNullGLContext.cpp b/src/gpu/gl/SkNullGLContext.cpp
index 86c09b2..576ee52 100644
--- a/src/gpu/gl/SkNullGLContext.cpp
+++ b/src/gpu/gl/SkNullGLContext.cpp
@@ -8,6 +8,9 @@
 
 #include "gl/SkNullGLContext.h"
 
-const GrGLInterface* SkNullGLContext::createGLContext() {
+const GrGLInterface* SkNullGLContext::createGLContext(GrGLStandard forcedGpuAPI) {
+    if (kGLES_GrGLStandard == forcedGpuAPI) {
+        return NULL;
+    }
     return GrGLCreateNullInterface();
 };