Abstract OSMesa from the Layout tests

ui/gl/gl_implementation.h

Index: ui/gl/gl_implementation.h
diff --git a/ui/gl/gl_implementation.h b/ui/gl/gl_implementation.h
index 23c96299e8eff7ecd3537b66dce14c6b9d785cd9..28cbde27188dc9618917c0b9bcd362a84bab1450 100644
--- a/ui/gl/gl_implementation.h
+++ b/ui/gl/gl_implementation.h
@@ -79,6 +79,9 @@ GL_EXPORT void SetGLImplementation(GLImplementation implementation);
 // Get the current GL implementation.
 GL_EXPORT GLImplementation GetGLImplementation();
 
+// Get the software GL implementation for the current platform.
+GL_EXPORT GLImplementation GetSoftwareGLImplementation();
+
 // Does the underlying GL support all features from Desktop GL 2.0 that were
 // removed from the ES 2.0 spec without requiring specific extension strings.
 GL_EXPORT bool HasDesktopGLFeatures();