Avoid reusing ShapeResult if the first character is unsafe

Generally, the first character of a |ShapeResult| is safe-to-
break, because it is an output of one shape operation.

However, when shaping across elements, for example:
  <span>A</span>V
we shape "AV" together, then split the |ShapeResult| to each
|LayoutText|. When between "A" and "V" is not safe-to-break,
the start of the |ShapeResult| for "V" is not safe-to-break,
and it needs to be reshaped when "A" changes.

This patch changes |NeedsShaping| logic so that a
|ShapeResult| whose start part is not safe-to-break should
be reshaped.

Bug: 1046095
Change-Id: I4a630514d14087f51c964896ba7503f648b7d93d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2700491
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Auto-Submit: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#855148}