[inspector] fixed one more size_t -> int conversion

This problem was detected only on linux64_gyp bot.
It's safe to convert length string into int from size_t.

R=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2402583004
Cr-Commit-Position: refs/heads/master@{#40101}

test/inspector/inspector-test.cc

@@ -157,12 +157,12 @@ v8::Local<v8::String> ToString(v8::Isolate* isolate,
   if (string.is8Bit())
     return v8::String::NewFromOneByte(isolate, string.characters8(),
                                       v8::NewStringType::kNormal,
-                                      string.length())
+                                      static_cast<int>(string.length()))
         .ToLocalChecked();
   else
     return v8::String::NewFromTwoByte(isolate, string.characters16(),
                                       v8::NewStringType::kNormal,
-                                      string.length())
+                                      static_cast<int>(string.length()))
         .ToLocalChecked();
 }