Pass the source string to ShapeResult functions

This patch changes OffsetForPosition and PositionForOffset
functions to accept its source string, and let callers to
pass them.

The given strings are not used yet in this patch, other than
DCHECK with the number of characters in the ShapeResult.

Computing grapheme cluster list for all ShapeResult turned
out to be not cheap enough for LayoutNG that this patch is
the preparation to compute them when needed.

Bug: 636993
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_layout_ng
Change-Id: I6dc22fda781f63219fba74381da87f1f0b33e85f
Reviewed-on: https://chromium-review.googlesource.com/1118062
Commit-Queue: Koji Ishii <kojii@chromium.org>
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#578525}
8 files changed