Merge to M50: Add a missing initialisation for CPDF_ContentMarkItem.

TBR=dsinclair@chromium.org
BUG=chromium:605491

Original Review: https://codereview.chromium.org/1910143002

Review URL: https://codereview.chromium.org/1915053006 .
diff --git a/core/src/fpdfapi/fpdf_page/fpdf_page_graph_state.cpp b/core/src/fpdfapi/fpdf_page/fpdf_page_graph_state.cpp
index de3a657..827b9b8 100644
--- a/core/src/fpdfapi/fpdf_page/fpdf_page_graph_state.cpp
+++ b/core/src/fpdfapi/fpdf_page/fpdf_page_graph_state.cpp
@@ -587,9 +587,8 @@
   }
   pGeneralState->m_Matrix = m_CTM;
 }
-CPDF_ContentMarkItem::CPDF_ContentMarkItem() {
-  m_ParamType = None;
-}
+CPDF_ContentMarkItem::CPDF_ContentMarkItem()
+    : m_ParamType(None), m_pParam(nullptr) {}
 CPDF_ContentMarkItem::CPDF_ContentMarkItem(const CPDF_ContentMarkItem& src) {
   m_MarkName = src.m_MarkName;
   m_ParamType = src.m_ParamType;