Bug 1313421 part 2: Use flex container's FlexLine linked-list to determine the first flex item, rather than its child-frame list. r=mats

This shouldn't change our behavior right now, but it will make a difference
after bug 1269045 -- when that bug lands, we'll have nsPlaceholderFrames (which
are not flex items) in the child-frame list, which means we can't depend on its
first entry being a flex item anymore.

MozReview-Commit-ID: KRXYaK8R8bc

layout/generic/nsFlexContainerFrame.cpp

@@ -837,6 +837,20 @@ public:
     return mItems.getFirst();
   }
 
+  FlexItem* GetLastItem()
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getLast();
+  }
+
+  const FlexItem* GetLastItem() const
+  {
+    MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
+               "mNumItems bookkeeping is off");
+    return mItems.getLast();
+  }
+
   bool IsEmpty() const
   {
     MOZ_ASSERT(mItems.isEmpty() == (mNumItems == 0),
@@ -4180,6 +4194,14 @@ nsFlexContainerFrame::DoFlexLayout(nsPresContext*           aPresContext,
   logSize += aReflowInput.ComputedLogicalBorderPadding().Size(flexWM);
   nsSize containerSize = logSize.GetPhysicalSize(flexWM);
 
+  // If the flex container has no baseline-aligned items, it will use this item
+  // (the first item, discounting any under-the-hood reversing that we've done)
+  // to determine its baseline:
+  const FlexItem* const firstItem =
+    aAxisTracker.AreAxesInternallyReversed()
+    ? lines.getLast()->GetLastItem()
+    : lines.getFirst()->GetFirstItem();
+
   // FINAL REFLOW: Give each child frame another chance to reflow, now that
   // we know its final size and position.
   for (const FlexLine* line = lines.getFirst(); line; line = line->getNext()) {
@@ -4230,10 +4252,10 @@ nsFlexContainerFrame::DoFlexLayout(nsPresContext*           aPresContext,
                        *item, framePos, containerSize);
       }
 
-      // If this is our first child and we haven't established a baseline for
+      // If this is our first item and we haven't established a baseline for
       // the container yet (i.e. if we don't have 'align-self: baseline' on any
       // children), then use this child's baseline as the container's baseline.
-      if (item->Frame() == mFrames.FirstChild() &&
+      if (item == firstItem &&
           flexContainerAscent == nscoord_MIN) {
         flexContainerAscent = itemNormalBPos + item->ResolvedAscent();
       }