Bug 779220. Add JS_InternStringN for callers that know the length of the string. r=luke

This mirrors the existing JS_InternUCStringN.

js/src/jsapi.cpp

@@ -5955,10 +5955,16 @@ JS_InternJSString(JSContext *cx, JSString *str)
 
 JS_PUBLIC_API(JSString *)
 JS_InternString(JSContext *cx, const char *s)
+{
+    return JS_InternStringN(cx, s, strlen(s));
+}
+
+JS_PUBLIC_API(JSString *)
+JS_InternStringN(JSContext *cx, const char *s, size_t length)
 {
     AssertHeapIsIdle(cx);
     CHECK_REQUEST(cx);
-    JSAtom *atom = js_Atomize(cx, s, strlen(s), InternAtom);
+    JSAtom *atom = js_Atomize(cx, s, length, InternAtom);
     JS_ASSERT_IF(atom, JS_StringHasBeenInterned(cx, atom));
     return atom;
 }

js/src/jsapi.h

@@ -5265,6 +5265,9 @@ JS_NewStringCopyZ(JSContext *cx, const char *s);
 extern JS_PUBLIC_API(JSString *)
 JS_InternJSString(JSContext *cx, JSString *str);
 
+extern JS_PUBLIC_API(JSString *)
+JS_InternStringN(JSContext *cx, const char *s, size_t length);
+
 extern JS_PUBLIC_API(JSString *)
 JS_InternString(JSContext *cx, const char *s);