Bug 1398404 - Make current context when updating max draw buffers; r=jgilbert

MozReview-Commit-ID: 1OSjjMwCpBb

--HG--
extra : rebase_source : 24f82404dcc4372f8b05e9778efa1d335ad786ad

dom/canvas/WebGLContext.cpp

@@ -2458,6 +2458,7 @@ WebGLContext::ValidateArrayBufferView(const char* funcName,
 void
 WebGLContext::UpdateMaxDrawBuffers()
 {
+    gl->MakeCurrent();
     mGLMaxColorAttachments = gl->GetIntAs<uint32_t>(LOCAL_GL_MAX_COLOR_ATTACHMENTS);
     mGLMaxDrawBuffers = gl->GetIntAs<uint32_t>(LOCAL_GL_MAX_DRAW_BUFFERS);