Abstract OSMesa from the Layout tests

ui/gl/gl_implementation.cc

Index: ui/gl/gl_implementation.cc
diff --git a/ui/gl/gl_implementation.cc b/ui/gl/gl_implementation.cc
index 73154b34e266b88cbc43cd57965ca78c6f89f553..9235cb935e55ff4823b11ede7bbc9d399f3d3396 100644
--- a/ui/gl/gl_implementation.cc
+++ b/ui/gl/gl_implementation.cc
@@ -80,6 +80,10 @@ GLImplementation GetNamedGLImplementation(const std::string& name) {
   return kGLImplementationNone;
 }
 
+GLImplementation GetSoftwareGLImplementation() {
+  return kGLImplementationOSMesaGL;
+}
+
 const char* GetGLImplementationName(GLImplementation implementation) {
   for (size_t i = 0; i < arraysize(kGLImplementationNamePairs); ++i) {
     if (implementation == kGLImplementationNamePairs[i].implementation)