[scanner] Regression test for Utf-8 BOM handling (crbug.com/685618).

The existing unit test explicitly checked for this case, but was - under
the right circumstances - defeated by the optimization to not re-run the
whole position search if we were close enough.

BUG=chromium:685618

Review-Url: https://codereview.chromium.org/2663883002
Cr-Commit-Position: refs/heads/master@{#42794}

test/cctest/parsing/test-scanner-streams.cc

@@ -132,6 +132,13 @@ TEST(Utf8StreamBOM) {
 
   stream->Seek(5);
   CHECK_EQ(unicode_ucs2[5], stream->Advance());
+
+  // Try again, but make sure we have to seek 'backwards'.
+  while (v8::internal::Utf16CharacterStream::kEndOfInput != stream->Advance()) {
+    // Do nothing. We merely advance the stream to the end of its input.
+  }
+  stream->Seek(5);
+  CHECK_EQ(unicode_ucs2[5], stream->Advance());
 }
 
 TEST(Utf8SplitBOM) {