[turbofan] Add benchmarks for String.prototype.charCodeAt to JSTests.json

Bug: v8:7127, v8:7092
Change-Id: Ib79c1cf5e65632b8701528799fe7df1d5407ad59
Reviewed-on: https://chromium-review.googlesource.com/846766
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50322}

test/js-perf-test/JSTests.json

@@ -188,7 +188,9 @@
       "tests": [
         {"name": "StringFunctions"},
         {"name": "StringIndexOfConstant"},
-        {"name": "StringIndexOfNonConstant"}
+        {"name": "StringIndexOfNonConstant"},
+        {"name": "StringCharCodeAtConstant"},
+        {"name": "StringCharCodeAtNonConstant"}
       ]
     },
     {