Use the extensions generated by FeatureInfo in the passthrough cmd decoder.

gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc
index 418388c9404ff38aa00e595d7d7ec92b21817e01..7cca1db60205530f36fcc85e927577b8fcac8139 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder_passthrough_doers.cc
@@ -1146,7 +1146,7 @@ error::Error GLES2DecoderPassthroughImpl::DoGetString(GLenum name,
       *result = GetServiceVendorString(feature_info_.get());
       break;
     case GL_EXTENSIONS:
-      *result = extension_string_.c_str();
+      *result = feature_info_->extensions().c_str();
       break;
     default:
       *result = reinterpret_cast<const char*>(glGetString(name));