No need to buffer CSS comments

Instead of looking for a slash and then looking behind for a star
in the output buffer, look for a star then look ahead for a slash.

Change-Id: I16ec56af8eeb27a972faebe08e34a11dad24d10f
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/66375
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

core/fxcrt/css/cfx_csssyntaxparser.cpp

diff --git a/core/fxcrt/css/cfx_csssyntaxparser.cpp b/core/fxcrt/css/cfx_csssyntaxparser.cpp
index eb32297..8c92de4 100644
--- a/core/fxcrt/css/cfx_csssyntaxparser.cpp
+++ b/core/fxcrt/css/cfx_csssyntaxparser.cpp
@@ -159,11 +159,9 @@
           }
           break;
         case SyntaxMode::kComment:
-          if (wch == '/' && m_Output.GetLength() > 0 &&
-              m_Output.GetBuffer()[m_Output.GetLength() - 1] == '*') {
+          if (wch == '*' && m_Input.GetNextChar() == '/') {
             RestoreMode();
-          } else {
-            m_Output.AppendChar(wch);
+            m_Input.MoveNext();
           }
           m_Input.MoveNext();
           break;