[esnext] implement StringPad spec changes from March TC39 meeting

Return the empty string from StringPad if the filler argument is the empty
string, effectively returning the string unchanged in StringPadStart and
StringPadEnd.

```
"foo".padStart(100000, "") -> "foo"
```

BUG=v8:4957, v8:4954
LOG=N
R=littledan@chromium.org

Review-Url: https://codereview.chromium.org/1926773003
Cr-Commit-Position: refs/heads/master@{#35837}

src/js/harmony-string-padding.js

@@ -30,7 +30,8 @@ function StringPad(thisString, maxLength, fillString) {
   } else {
     fillString = TO_STRING(fillString);
     if (fillString === "") {
-      fillString = " ";
+      // If filler is the empty String, return S.
+      return "";
     }
   }
 

test/mjsunit/harmony/harmony-string-pad-end.js

@@ -67,8 +67,19 @@
 (function TestFillerToString() {
   assertEquals(".         ", ".".padEnd(10));
   assertEquals(".         ", ".".padEnd(10, undefined));
-  assertEquals(".         ", ".".padEnd(10, { toString() { return ""; } }));
   assertEquals(".nullnulln", ".".padEnd(10, null));
+  assertEquals(".XXXXXXXXX", ".".padEnd(10, { toString() { return "X"; } }));
+  assertEquals(
+      ".111111111",
+      ".".padEnd(10, { toString: undefined, valueOf() { return 1; } }));
+})();
+
+
+(function TestFillerEmptyString() {
+  assertEquals(".", ".".padEnd(10, ""));
+  assertEquals(".", ".".padEnd(10, { toString() { return ""; } }));
+  assertEquals(
+      ".", ".".padEnd(10, { toString: undefined, valueOf() { return ""; } }));
 })();
 
 

test/mjsunit/harmony/harmony-string-pad-start.js

@@ -67,8 +67,19 @@
 (function TestFillerToString() {
   assertEquals("         .", ".".padStart(10));
   assertEquals("         .", ".".padStart(10, undefined));
-  assertEquals("         .", ".".padStart(10, { toString() { return ""; } }));
   assertEquals("nullnulln.", ".".padStart(10, null));
+  assertEquals("XXXXXXXXX.", ".".padStart(10, { toString() { return "X"; } }));
+  assertEquals(
+      "111111111.",
+      ".".padStart(10, { toString: undefined, valueOf() { return 1; } }));
+})();
+
+
+(function TestFillerEmptyString() {
+  assertEquals(".", ".".padStart(10, ""));
+  assertEquals(".", ".".padStart(10, { toString() { return ""; } }));
+  assertEquals(
+      ".", ".".padStart(10, { toString: undefined, valueOf() { return ""; } }));
 })();