Simplify LayoutObject::IsSelected

It turned out that |LayoutObject:IsSelected()| is not as fast
as it was expected, when used in very hot functions. Maybe it
could be inlined in the official build, but keeping this fast
and small helps running perf tests at least.

This patch also turns |IsTextFragment()| into a bit-field, to
avoid a virtual call which prevents inlining fast code path.

This patch is to help LayoutNG pre-paint faster.

Bug: 636993
Change-Id: I42223c2ee954fe5fc25e2c3fff0c07f639ee6dd7
Reviewed-on: https://chromium-review.googlesource.com/c/1460576
Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#630217}
5 files changed