[Selection API] implement getComposedRanges()

Add selection.getComposedRanges() which returns an array of
0 or 1 StaticRange. This patch only implements the steps from the
spec [1]. This patch does not update the cross shadow boundaries check.
That will be fixed in a future patch.

[1] https://w3c.github.io/selection-api/#dom-selection-getcomposedranges

Change-Id: I65cd9a272ea00897a44fbc13eef97983c3e29005
Bug: 40286116
Bug: 335245347
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5734507
Commit-Queue: Di Zhang <dizhangg@chromium.org>
Reviewed-by: Mason Freed <masonf@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1333845}