[LayoutNG] Ensure ShapingLineBreaker moves offset forward

After the fix for crbug.com/915567, there are reports of
cases where ShapingLineBreaker does not move forward.

While I could not find a reproducible case for this, there
is a case it may occur from the code review. This patch fixes
the case, along with adding CHECK to ShapingLineBreaker.

This is a speculative fix.

Bug: 928178
Change-Id: I51cf733fdad9abf2feee529bfaf90a7e1bdbb09d
Reviewed-on: https://chromium-review.googlesource.com/c/1453777
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629151}

third_party/blink/renderer/platform/fonts/shaping/shaping_line_breaker.cc

@@ -255,7 +255,8 @@ scoped_refptr<const ShapeResultView> ShapingLineBreaker::ShapeLine(
       return ShapeToEnd(start, first_safe, range_start, range_end);
     }
   }
-  DCHECK_GT(break_opportunity.offset, start);
+  // It is critical to move forward, or callers may end up in an infinite loop.
+  CHECK_GT(break_opportunity.offset, start);
 
   // If the start offset is not at a safe-to-break boundary the content between
   // the start and the next safe-to-break boundary needs to be reshaped and the
@@ -281,7 +282,6 @@ scoped_refptr<const ShapeResultView> ShapingLineBreaker::ShapeLine(
 
   scoped_refptr<const ShapeResult> line_end_result;
   unsigned last_safe = break_opportunity.offset;
-  if (break_opportunity.offset > start) {
   // If the previous valid break opportunity is not at a safe-to-break
   // boundary reshape between the safe-to-break offset and the valid break
   // offset. If the resulting width exceeds the available space the
@@ -321,10 +321,19 @@ scoped_refptr<const ShapeResultView> ShapingLineBreaker::ShapeLine(
     // because none can fit. The one after candidate_break is better for
     // ligatures, but the one before is better for kernings.
     break_opportunity = PreviousBreakOpportunity(candidate_break, start);
+    if (break_opportunity.offset <= start) {
+      break_opportunity = NextBreakOpportunity(
+          std::max(candidate_break, start + 1), start, range_end);
+      if (break_opportunity.offset >= range_end) {
+        result_out->break_offset = range_end;
+        return ShapeToEnd(start, first_safe, range_start, range_end);
+      }
+    }
     // Loop once more to compute last_safe for the new break opportunity.
     is_overflow = true;
   }
-  }
+  // It is critical to move forward, or callers may end up in an infinite loop.
+  CHECK_GT(break_opportunity.offset, start);
   DCHECK_GE(break_opportunity.offset, last_safe);
   DCHECK_EQ(break_opportunity.offset - start,
             (line_start_result ? line_start_result->NumCharacters() : 0) +
@@ -344,7 +353,6 @@ scoped_refptr<const ShapeResultView> ShapingLineBreaker::ShapeLine(
     segments[count++] = {line_end_result.get(), last_safe, max_length};
   auto line_result = ShapeResultView::Create(&segments[0], count);
 
-  DCHECK_GT(break_opportunity.offset, start);
   DCHECK_LE(break_opportunity.offset, range_end);
   DCHECK_EQ(break_opportunity.offset - start, line_result->NumCharacters());