* 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
#include "third_party/blink/public/web/web_script_source.h"
#include "third_party/blink/public/web/web_view.h"
#include "url/url_util.h"
#include "atom/common/node_includes.h"
namespace mate {
template <>
struct Converter<blink::WebLocalFrame::ScriptExecutionType> {
  static bool FromV8(v8::Isolate* isolate,
                     v8::Local<v8::Value> val,
                     blink::WebLocalFrame::ScriptExecutionType* out) {
    std::string execution_type;
    if (!ConvertFromV8(isolate, val, &execution_type))
      return false;
    if (execution_type == "asynchronous") {
      *out = blink::WebLocalFrame::kAsynchronous;
    } else if (execution_type == "asynchronousBlockingOnload") {
      *out = blink::WebLocalFrame::kAsynchronousBlockingOnload;
    } else if (execution_type == "synchronous") {
      *out = blink::WebLocalFrame::kSynchronous;
    } else {
      return false;
    }
    return true;
  }
};
}  
namespace atom {
namespace api {
namespace {
content::RenderFrame* GetRenderFrame(v8::Local<v8::Value> value) {