[New Multicolumn] flowThreadTranslationAtOffset() needs to be relative to the flow thread.

[New Multicolumn] flowThreadTranslationAtOffset() needs to be relative to the flow thread.

When we want to translate from flow thread coordinates to visual coordinates,
we need to return something relative to the top/left of the flow thread, not
something relative to the top/left of the content box of the multicol
container. This distinction matters in RTL mode, since the flow thread is then
positioned at the top/right inside the multicol container.

Added LayoutTests/fast/multicol/client-rects-rtl.html , which is an adaption
for RTL of client-rects.html for RTL. This didn't pass prior to this fix. Also
added an assertion failure test, which is what the original bug report was
about.

BUG=479155
R=dsinclair@chromium.org,jchaffraix@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=194517

[dsinclair, 2015-04-27 14:42:07]

The CQ bit was checked by dsinclair@chromium.org

[dsinclair, 2015-04-27 14:42:08]

lgtm

[commit-bot: I haz the power, 2015-04-27 14:42:19]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1102273004/1

[commit-bot: I haz the power, 2015-04-27 15:05:34]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=194517