Enable creation of offscreen contexts which own their backing surface.

ui/gl/gl_context_egl.cc

Index: ui/gl/gl_context_egl.cc
diff --git a/ui/gl/gl_context_egl.cc b/ui/gl/gl_context_egl.cc
index 1cd08cd40496efa08ee512b4dfe85efcc9d69296..6e85155f8347be950b51b2829b134f4308013990 100644
--- a/ui/gl/gl_context_egl.cc
+++ b/ui/gl/gl_context_egl.cc
@@ -32,6 +32,12 @@ extern "C" {
 #define EGL_CONTEXT_WEBGL_COMPATIBILITY_ANGLE 0x3AAC
 #endif /* EGL_ANGLE_create_context_webgl_compatibility */
 
+#ifndef EGL_CONTEXT_PRIORITY_LEVEL_IMG
+#define EGL_CONTEXT_PRIORITY_LEVEL_IMG 0x3100
+#define EGL_CONTEXT_PRIORITY_LOW_IMG 0x3103
+#endif /* EGL_CONTEXT_PRIORITY_LEVEL */
+
+
 using ui::GetLastEGLErrorString;
 
 namespace gl {
@@ -105,6 +111,12 @@ bool GLContextEGL::Initialize(GLSurface* compatible_surface,
     DCHECK(!attribs.webgl_compatibility_context);
   }
 
+  if (GLSurfaceEGL::IsEGLContextPrioritySupported() && attribs.low_priority) {
+    DVLOG(1) << __FUNCTION__ << ": setting low priority";
+    context_attributes.push_back(EGL_CONTEXT_PRIORITY_LEVEL_IMG);
+    context_attributes.push_back(EGL_CONTEXT_PRIORITY_LOW_IMG);
+  }
+
   // Append final EGL_NONE to signal the context attributes are finished
   context_attributes.push_back(EGL_NONE);
   context_attributes.push_back(EGL_NONE);