Make input-implicit-length-limit.html faster

The test doesn't need to handle strings consisted of control characters.
This CL makes the test 4x faster in debug build.

This CL has no behavior changes.

Bug: 1143185
Change-Id: I82c252ae3b182514cb11543a3c305a589a6366d1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2503763
Commit-Queue: Kent Tamura <tkent@chromium.org>
Auto-Submit: Kent Tamura <tkent@chromium.org>
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821649}

third_party/blink/web_tests/fast/forms/text/input-implicit-length-limit.html

@@ -12,7 +12,7 @@ function attempt(length, expected)
 {
     var buffer = new Uint8Array(new ArrayBuffer(length));
     for (var i = 0; i < buffer.length; ++i)
-        buffer[i] = i % 10;
+        buffer[i] = 0x30 + (i % 10);
     var testString = (new TextDecoder("utf-8")).decode(buffer);
 
     input.value = testString;