SegmentedString: Use decrementAndCheckLength where possible

Source/platform/text/SegmentedString.h

Index: Source/platform/text/SegmentedString.h
diff --git a/Source/platform/text/SegmentedString.h b/Source/platform/text/SegmentedString.h
index 88ffb605caae433f8f3e524b58b758c778e92196..fe4e288b98f24248a490227f3b39c18f25bbf505 100644
--- a/Source/platform/text/SegmentedString.h
+++ b/Source/platform/text/SegmentedString.h
@@ -219,12 +219,7 @@ public:
         if (m_fastPathFlags & Use8BitAdvance) {
             ASSERT(!m_pushedChar1);
             m_currentChar = m_currentString.incrementAndGetCurrentChar8();
-            m_currentString.decrementLength();
-            if (!m_currentString.haveOneCharacterLeft())
-                return;
-
-            updateSlowCaseFunctionPointers();
-
+            decrementAndCheckLength();
             return;
         }
 
@@ -238,17 +233,13 @@ public:
 
             bool haveNewLine = (m_currentChar == '\n') & !!(m_fastPathFlags & Use8BitAdvanceAndUpdateLineNumbers);
             m_currentChar = m_currentString.incrementAndGetCurrentChar8();
-            m_currentString.decrementLength();
-            bool haveOneCharacterLeft = m_currentString.haveOneCharacterLeft();
+            decrementAndCheckLength();
 
             if (haveNewLine) {

[tkent, 2015/08/27 03:08:01]

This block doesn't refer to data members updated by
updateSlowCaseFunctionPointers().  So, the change should be safe.

                 ++m_currentLine;
                 m_numberOfCharactersConsumedPriorToCurrentLine =  m_numberOfCharactersConsumedPriorToCurrentString + m_currentString.numberOfCharactersConsumed();
             }
 
-            if (haveOneCharacterLeft)
-                updateSlowCaseFunctionPointers();
-
             return;
         }