[Live Caption] Speculative fix of a crash in GetNumLinesInLabel.

An EXCEPTION_ACCESS_VIOLATION_READ crash was reported in
CaptionBubble::GetNumLinesInLabel. I believe the cause is that
CaptionBubble::GetNumLinesInLabel calls Label::GetRequiredLines, which
is a const function. To fix this, I mark
CaptionBubble::GetNumLinesInLabel as const.

While I am here, I am also performing a check that observer_ is not
null in CaptionBubbleModel::CommitPartialText, which would cause a
different crash.

Bug: 1055150, 1096366
Change-Id: Ic039de789f4ce5dc34b3dff0ee11e38bcfabb4b4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2252198
Commit-Queue: Abigail Klein <abigailbklein@google.com>
Reviewed-by: Katie Dektar <katie@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#780003}

chrome/browser/ui/views/accessibility/caption_bubble.cc

@@ -536,7 +536,7 @@ size_t CaptionBubble::GetTextIndexOfLineInLabel(size_t line) const {
   return label_->GetTextIndexOfLine(line);
 }
 
-size_t CaptionBubble::GetNumLinesInLabel() {
+size_t CaptionBubble::GetNumLinesInLabel() const {
   return label_->GetRequiredLines();
 }
 

chrome/browser/ui/views/accessibility/caption_bubble.h

@@ -60,7 +60,7 @@ class CaptionBubble : public views::BubbleDialogDelegateView,
   size_t GetTextIndexOfLineInLabel(size_t line) const;
 
   // Returns the number of lines in the caption bubble label that are rendered.
-  size_t GetNumLinesInLabel();
+  size_t GetNumLinesInLabel() const;
 
   const char* GetClassName() const override;
 

chrome/browser/ui/views/accessibility/caption_bubble_model.cc

@@ -89,12 +89,15 @@ void CaptionBubbleModel::CommitPartialText() {
   }
   partial_text_.clear();
 
+  if (!observer_)
+    return;
+
   // Truncate the final text to kMaxLines lines long. This time, alert the
   // observer that the text has changed.
   const size_t num_lines = observer_->GetNumLinesInLabel();
   if (num_lines > kMaxLines) {
     DCHECK(base::IsStringASCII(final_text_));
-    size_t truncate_index =
+    const size_t truncate_index =
         observer_->GetTextIndexOfLineInLabel(num_lines - kMaxLines);
     final_text_.erase(0, truncate_index);
     OnTextChange();