[LayoutNG] Add "self-paint" to ShowFragmentTree output

Now the tree looks like this:
 Box (block-flow children-inline) offset:0,34 size:600x72
    LineBox offset:31,0 size:568x18
      Box (children-inline)(self paint) offset:0,-1 size:367x19
        Text offset:0,1 size:337x17 start: 30 end: 83
      Text offset:377,0 size:191x17 start: 1 end: 30
    LineBox offset:24,18 size:575x18
      Box (children-inline)(self paint) offset:0,-1 size:575x19
        Text offset:0,1 size:200x17 start: 144 end: 171
        Text offset:200,1 size:375x17 start: 84 end: 144
    LineBox offset:21,36 size:578x18

Useful when debugging painting.

Bug: 635619
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_layout_ng
Change-Id: I0901841736b7bbad9534a3ecbcc187c9b968d15c
Reviewed-on: https://chromium-review.googlesource.com/892103
Commit-Queue: Aleks Totic <atotic@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532788}
2 files changed