[css-flexbox] Add comment to IntrinsicContentBlockSize

As requested in https://crrev.com/c/1546002

R=dgrogan@chromium.org

Bug: 945214
Change-Id: I20f6d82971b3a1b4ab632f7ac30db56ff792c36f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1546938
Auto-Submit: Christian Biesinger <cbiesinger@chromium.org>
Commit-Queue: David Grogan <dgrogan@chromium.org>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#646610}

third_party/blink/renderer/core/layout/flexible_box_algorithm.h

@@ -314,6 +314,13 @@ class FlexLayoutAlgorithm {
 
   bool ShouldApplyMinSizeAutoForChild(const LayoutBox& child) const;
 
+  // Returns the intrinsic size of this box in the block direction. Call this
+  // after all flex lines have been created and processed (ie. after the
+  // ComputeLineItemsPosition stage).
+  // For a column flexbox, this will return the max across all flex lines of
+  // the length of the line, minus any added spacing due to justification.
+  // For row flexboxes, this returns the bottom (block axis) of the last flex
+  // line. In both cases, border/padding is not included.
   LayoutUnit IntrinsicContentBlockSize() const;
 
   static TransformedWritingMode GetTransformedWritingMode(const ComputedStyle&);