Fix LayoutTest transitions/flex-transitions.html

Update the expected results to account for the fact that flex-grow and
flex-shrink don't interpolate smoothly to or from zero. Also update the test to
use values of similar magnitudes for each part of the property shorthand, to
allow the tolerance to be applied meaningfully.

Note that interpolation of this property is tested more thoroughly in
animations/interpolation/flex-interpolation.html.

R=timloh@chromium.org

Review URL: https://codereview.chromium.org/51563003

git-svn-id: svn://svn.chromium.org/blink/trunk@160904 bbb929c8-8fbe-4397-9dbb-9b2b20218538