* Output of `node_modules/.bin/electron --version`: v3.0.5
* Operating System (Platform and Version): macOS 10.14

**Expected Behavior**
When webPreferences.sandbox is set to true and webFrame is using a provider for spell checking, the memory must be released after page reload.

**Actual behavior**
The memory isn't released and caused memory leak

**To Reproduce**

- Clone https://github.com/vrbsm/example-memory-leak-electron
- Run npm run start
Memory usage of electron process will be increasing in task manager after reloading the page several times

**Screenshots**
![test-memory-leak-01](https://user-images.githubusercontent.com/9887288/47675838-c2d4b800-db99-11e8-97e8-b5b27bcab929.gif)



atom/renderer/api/atom_api_web_frame.cc
v8::Local<v8::FunctionTemplate> prototype) {
  prototype->SetClassName(mate::StringToV8(isolate, "WebFrame"));
  mate::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .SetMethod("setName", &WebFrame::SetName)
      .SetMethod("setZoomLevel", &WebFrame::SetZoomLevel)
      .SetMethod("getZoomLevel", &WebFrame::GetZoomLevel)
      .SetMethod("setZoomFactor", &WebFrame::SetZoomFactor)
      .SetMethod("getZoomFactor", &WebFrame::GetZoomFactor)
      .SetMethod("setVisualZoomLevelLimits",
                 &WebFrame::SetVisualZoomLevelLimits)
      .SetMethod("setLayoutZoomLevelLimits",
                 &WebFrame::SetLayoutZoomLevelLimits)
      .SetMethod("allowGuestViewElementDefinition",
                 &WebFrame::AllowGuestViewElementDefinition)
      .SetMethod("getWebFrameId", &WebFrame::GetWebFrameId)
      .SetMethod("setSpellCheckProvider", &WebFrame::SetSpellCheckProvider)
      .SetMethod("insertText", &WebFrame::InsertText)
      .SetMethod("insertCSS", &WebFrame::InsertCSS)
      .SetMethod("executeJavaScript", &WebFrame::ExecuteJavaScript)
      .SetMethod("executeJavaScriptInIsolatedWorld",
                 &WebFrame::ExecuteJavaScriptInIsolatedWorld)
      .SetMethod("setIsolatedWorldSecurityOrigin",
                 &WebFrame::SetIsolatedWorldSecurityOrigin)
      .SetMethod("setIsolatedWorldContentSecurityPolicy",
                 &WebFrame::SetIsolatedWorldContentSecurityPolicy)
      .SetMethod("setIsolatedWorldHumanReadableName",
                 &WebFrame::SetIsolatedWorldHumanReadableName)