[LayoutNG] Update NGInlineLayoutAlgorithm class documentation.

This algorithm is now only invoked when laying out an inline formatting
context root, not at every inline layout box (such as SPAN, B, etc.).

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I789c424310636292ebca4aa117683e562ff9a318
Reviewed-on: https://chromium-review.googlesource.com/828873Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524366}

third_party/WebKit/Source/core/layout/ng/inline/ng_inline_layout_algorithm.h

@@ -27,7 +27,8 @@ class NGLineBoxFragmentBuilder;
 class NGTextFragmentBuilder;
 struct NGPositionedFloat;
 
-// A class for inline layout (e.g. a <span> with no special style).
+// A class for laying out an inline formatting context, i.e. a block with inline
+// children.
 //
 // This class determines the position of NGInlineItem and build line boxes.
 //