Fix double to integer conversions in runtime string indexing.

test/mjsunit/string-charat.js

Index: test/mjsunit/string-charat.js
diff --git a/test/mjsunit/string-charat.js b/test/mjsunit/string-charat.js
index 99afb75c1e62f2b044cb149e616e8123a34e140d..5ce4e89473b52ae3826943c0515e9119e2694cc0 100644
--- a/test/mjsunit/string-charat.js
+++ b/test/mjsunit/string-charat.js
@@ -48,6 +48,7 @@ function basicTest(s, len) {
   assertEquals("", s().charAt(-1/0));
   assertEquals("t", s().charAt(0));
   assertEquals("t", s().charAt(-0.0));
+  assertEquals("t", s().charAt(-0.1));
   assertEquals("t", s().charAt(0.4));
   assertEquals("e", s().charAt(slowIndex1));
   assertEquals("s", s().charAt(slowIndex2));
@@ -63,6 +64,7 @@ function basicTest(s, len) {
   assertEquals(101, s().charCodeAt(true));
   assertEquals(116, s().charCodeAt(0));
   assertEquals(116, s().charCodeAt(-0.0));
+  assertEquals(116, s().charCodeAt(-0.1));
   assertEquals(116, s().charCodeAt(0.4));
   assertEquals(101, s().charCodeAt(slowIndex1));
   assertEquals(115, s().charCodeAt(slowIndex2));