Store and propagate a list of applied text decorations.

This is preparatory work to fix an issue with style recalculation of text
decorations (350840), and to implement multiple text decorations per line.

In a future patch, the color and style of each decoration will be added to
the AppliedTextDecoration objects in order to fix 350840.

BUG=350840, 315271
TEST=Covered by existing tests

Review URL: https://codereview.chromium.org/272443002

git-svn-id: svn://svn.chromium.org/blink/trunk@173761 bbb929c8-8fbe-4397-9dbb-9b2b20218538