Use correct type for profile script ID

Summary:
Stopping the sampling profiler produced a JSON trace with
numeric `scriptId`s, whereas [the CDP spec expects a string](https://chromedevtools.github.io/devtools-protocol/tot/Runtime/#type-ScriptId),
resulting in an exception instead of a profiler trace:
```
JSONValue not a string
```

Make the `ChromeTraceSerializer` emit strings for `scriptID` to
adhere to the protocol spec.

Reviewed By: dannysu

Differential Revision: D48325992

fbshipit-source-id: a23c3acafd6c8f3d80bf8f75d94de5e21f7256db

lib/VM/Profiler/ChromeTraceSerializer.cpp

@@ -376,7 +376,7 @@ static void emitProfileNode(
 
   json.openDict();
   json.emitKeyValue("functionName", name);
-  json.emitKeyValue("scriptId", scriptId);
+  json.emitKeyValue("scriptId", std::to_string(scriptId));
   json.emitKeyValue("url", url);
   json.emitKeyValue("lineNumber", lineNumber);
   json.emitKeyValue("columnNumber", columnNumber);