Don't run transitions to/from 'auto' values

Don't run transitions to/from 'auto' values
https://bugs.webkit.org/show_bug.cgi?id=38243
rdar://problem/20904225

Reviewed by Dean Jackson.
Source/WebCore:

The specified behavior of transitions and animations is to not animate when either endpoint is 'auto'.
Previously, we were treating 'auto' as zero and interpolating, which caused unwanted animations
on fandango.com, airbnb.com and others.

Fix by having blend() return the from value if progress is 0, otherwise the to value. The
isZero() check can be removed, since this was effectively picking up auto value for one endpoint.

Tests: transitions/lengthsize-transition-to-from-auto.html
       transitions/transition-to-from-auto.html

* platform/Length.cpp:
(WebCore::blend):

LayoutTests:

* platform/mac-wk2/transitions/default-timing-function-expected.txt: New baseline.
* transitions/default-timing-function.html: Set explicit start value.
* transitions/interrupted-all-transition.html: Set explicit start value.
* transitions/lengthsize-transition-to-from-auto-expected.txt: Added.
* transitions/lengthsize-transition-to-from-auto.html: Added.
* transitions/transition-to-from-auto-expected.txt: Added.
* transitions/transition-to-from-auto.html: Added.
* transitions/zero-duration-in-list.html: Set explicit start value.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@200360 268f45cc-cd09-0410-ab3c-d52691b4dbfc