Make inline blocks correctly update their line box selection state.

When text is selected, RenderText::setSelectionState marks each RootInlineBox as having selected children.  But if a line contained only an inline-block element, the selection state of that line's RootInlineBox was not updated.

That caused RenderBlockFlow::inlineSelectionGaps to set lastLogicalTop to the wrong value, since it believed the line was not selected.

That caused RenderBlock::selectionGaps to paint a vertical selection gap on top of the affected line, obscuring its content.

This patch adds RenderBlock::setSelectionState, which updates the selection state of the RootInlineBox.

BUG=388324

Review URL: https://codereview.chromium.org/331373006

git-svn-id: svn://svn.chromium.org/blink/trunk@176944 bbb929c8-8fbe-4397-9dbb-9b2b20218538