Fix RenderBlock::percentHeightDescendants()

RenderBlock::percentHeightDescendants() is used to schedule layout of
descencents of a block when the logical height of the block changes.
It should contain descendants having percent height whose real height
calculation depends on the height of the block.

Previously
- the set doesn't contain direct children;
- skipped containing blocks for percent height calculation unnecessarily
  have the set.

Let the set contain all necessary objects. This doesn't fix any
functional breakage for now, but later layout optimization may depend
on the correct behavior of the function.

BUG=327815
TEST=WebFrameTest.RenderBlockPercentHeightDescendants

Review URL: https://codereview.chromium.org/201153008

git-svn-id: svn://svn.chromium.org/blink/trunk@169588 bbb929c8-8fbe-4397-9dbb-9b2b20218538