Bug 1484126 - part 16: Replace |CellData::mEffectiveColSpan - 1| with CellData::NumberOfFollowingColumns() r=m_kato

Differential Revision: https://phabricator.services.mozilla.com/D8353

--HG--
extra : moz-landing-system : lando

editor/libeditor/HTMLTableEditor.cpp

@@ -2590,7 +2590,7 @@ HTMLEditor::JoinTableCells(bool aMergeNonContiguousContents)
           }
           // Save max selected column in this row, including extra colspan
           lastColInRow =
-            cellData.mCurrent.mColumn + cellData.mEffectiveColSpan - 1;
+            cellData.mCurrent.mColumn + cellData.NumberOfFollowingColumns();
           cellFoundInRow = true;
         } else if (cellFoundInRow) {
           // No cell or not selected, but at least one cell in row was found