[PDF] Avoid unneeded cache invalidation in PDFiumRange::SetCharCount()

Currently, PDFiumRange::SetCharCount() will unconditionally mark the
PDFiumRange instance's cached screen rects as dirty. This is unnecessary
if the character count has not changed. Check for this condition and
turn SetCharCount() into a no-op.

Change-Id: I699d916b155d8e237774d7b408474e304bd07daa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6230982
Reviewed-by: Andy Phan <andyphan@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1416281}

pdf/pdfium/pdfium_range.cc

@@ -63,6 +63,10 @@ PDFiumRange& PDFiumRange::operator=(PDFiumRange&&) noexcept = default;
 PDFiumRange::~PDFiumRange() = default;
 
 void PDFiumRange::SetCharCount(int char_count) {
+  if (char_count == char_count_) {
+    return;
+  }
+
   char_count_ = char_count;
 #if DCHECK_IS_ON()
   int dummy_index = 0;