Fix three bugs in the equals() implementations for css gradients.

Fix three bugs in the equals() implementations for css gradients.

1. Linear gradients were considered equal if the first gradient has no x and y
   position and the second has no x but does have y.
2. Same as 1, for radial gradients. (This doesn't happen in practice as
   CSSParser::parseRadialGradient rejects such input, so no test for this case.)
3. Radial gradients without x and y position weren't considered equal even if
   they were.

Found by clang's -Wunused-value in combination with
https://codereview.chromium.org/26205004/

BUG=none
R=eae@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=159903

[eae, 2013-10-17 19:30:06]

LGTM

[commit-bot: I haz the power, 2013-10-17 19:38:02]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/thakis@chromium.org/26147006/17001

[Nico, 2013-10-17 22:16:00]

Committed patchset #4 manually as r159903 (presubmit successful).