Shape the following string with kRtl:
UChar string[] = {kZeroWidthSpaceCharacter,
                    kZeroWidthSpaceCharacter,
                    0x0627,
                    0x0631,
                    0x062F,
                    0x0648,
                    kZeroWidthSpaceCharacter,
                    kZeroWidthSpaceCharacter};

1. mac_chromium_rel_ng (10.9, it seems)
#chars=8, #glyphs=4, dir=1,
runs[1]{
  0:{start=2, #chars=6, dir=5, glyphs[4]{
    0:{char=3, glyph=356}
    1:{char=2, glyph=268}
    2:{char=1, glyph=272}
    3:{char=0, glyph=240}}}}

2. my MacBook (10.12.6)
#chars=8, #glyphs=5, dir=1,
runs[2]{
  0:{start=2, #chars=6, dir=5, glyphs[4]{
    0:{char=3, glyph=356}
    1:{char=2, glyph=268}
    2:{char=1, glyph=272}
    3:{char=0, glyph=240}}}
  1:{start=0, #chars=2, dir=5, glyphs[1]{
    0:{char=0, glyph=65535}}}}

3. Linux
#chars=8, #glyphs=8, dir=1,
runs[1]{
  0:{start=0, #chars=8, dir=5, glyphs[8]{
    0:{char=7, glyph=3}
    1:{char=6, glyph=3}
    2:{char=5, glyph=1392}
    3:{char=4, glyph=1372}
    4:{char=3, glyph=1374}
    5:{char=2, glyph=1364}
    6:{char=1, glyph=3}
    7:{char=0, glyph=3}}}}

Differences:
A. On mac bots, we're missing glyph_data for the zwsp. We rely on start_index of the last run (in RTL) to know the start range, and this is blocking issue 722462 (and will affect LayoutNG line breaker.)
B. On my local MacBook, start_index is 0, but character index 1 is missing.