Rename GetMargin to GetMarginIfExists

This CL makes it explicit that GetMargin may return a nullptr.

Change-Id: I46780e8df9e966accc38baf56f8aa5149533db87
Reviewed-on: https://pdfium-review.googlesource.com/22675
Reviewed-by: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/xfa/fxfa/cxfa_ffrectangle.cpp b/xfa/fxfa/cxfa_ffrectangle.cpp
index f3f2a5a..93c4ea2 100644
--- a/xfa/fxfa/cxfa_ffrectangle.cpp
+++ b/xfa/fxfa/cxfa_ffrectangle.cpp
@@ -24,7 +24,7 @@
     return;
 
   CFX_RectF rect = GetRectWithoutRotate();
-  CXFA_Margin* margin = m_pNode->GetMargin();
+  CXFA_Margin* margin = m_pNode->GetMarginIfExists();
   if (margin)
     XFA_RectWithoutMargin(rect, margin);