Set MaxDrawBuffers to 1 if WEBGL_draw_buffers is not enabled in WebGL.

gpu/command_buffer/service/gles2_cmd_decoder.cc

Index: gpu/command_buffer/service/gles2_cmd_decoder.cc
diff --git a/gpu/command_buffer/service/gles2_cmd_decoder.cc b/gpu/command_buffer/service/gles2_cmd_decoder.cc
index be644ce11ef3c4088bf6f42fa6eb03582a9d01c1..1489fc64ab1abe497db73b4b6bf6ea626c07cc09 100644
--- a/gpu/command_buffer/service/gles2_cmd_decoder.cc
+++ b/gpu/command_buffer/service/gles2_cmd_decoder.cc
@@ -2483,6 +2483,8 @@ bool GLES2DecoderImpl::InitializeShaderTranslator() {
     resources.OES_standard_derivatives = derivatives_explicitly_enabled_;
     resources.EXT_frag_depth = frag_depth_explicitly_enabled_;
     resources.EXT_draw_buffers = draw_buffers_explicitly_enabled_;
+    if (!draw_buffers_explicitly_enabled_)
+      resources.MaxDrawBuffers = 1;
   } else {
     resources.OES_standard_derivatives =
         features().oes_standard_derivatives ? 1 : 0;