Allow textures to be moved from one GL context group to another.

gpu/command_buffer/client/gles2_implementation.cc

Index: gpu/command_buffer/client/gles2_implementation.cc
===================================================================
--- gpu/command_buffer/client/gles2_implementation.cc	(revision 133771)
+++ gpu/command_buffer/client/gles2_implementation.cc	(working copy)
@@ -3138,5 +3138,20 @@
 #endif
 }
 
+void GLES2Implementation::GenMailboxCHROMIUM(
+    GLchar* mailbox) {
+  GPU_CLIENT_SINGLE_THREAD_CHECK();
+  GPU_CLIENT_LOG("[" << this << "] glGenMailboxCHROMIUM("
+      << static_cast<const void*>(mailbox) << ")");
+  TRACE_EVENT0("gpu", "GLES2::GenMailboxCHROMIUM");
+
+  helper_->GenMailboxCHROMIUM(kResultBucketId);
+
+  std::vector<int8> result;
+  GetBucketContents(kResultBucketId, &result);
+
+  std::copy(result.begin(), result.end(), mailbox);
+}
+
 }  // namespace gles2
 }  // namespace gpu