Don't assert when serializing -mask property.

Don't assert when serializing -mask property.

In the case -webkit-mask property contains several layers, it is possible
that one of the layer contains "initial" implicitly set simply because the
repeat property was not specified. However when reconstructing the
mask property from StylePropertySet it is possible to have for example
-mask-repeat-x with a value like 'initial, repeat-x' where the first initial
is implicit (the original -mask first layer did not set it). As
StylePropertySet should not return implicitly set initial values we should
just omit these ones. The ASSERT was triggered because of better checks before
casting to CSSPrimitiveValues but in this example we should not reach that
cast, we should bail out : 'initial' is not a primitive value.

BUG=242931
R=apavlov@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=151074

[apavlov, 2013-05-23 18:46:34]

Thanks a bunch! LGTM

[commit-bot: I haz the power, 2013-05-23 18:55:40]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexis.menard@intel.com/15912003/4001

[commit-bot: I haz the power, 2013-05-23 20:39:46]

Retried try job too often on linux_layout_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_layo...

[commit-bot: I haz the power, 2013-05-23 20:43:21]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexis.menard@intel.com/15912003/4001

[commit-bot: I haz the power, 2013-05-23 22:23:29]

Retried try job too often on mac_layout_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=mac_layout...

[commit-bot: I haz the power, 2013-05-24 00:21:52]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexis.menard@intel.com/15912003/4001

[commit-bot: I haz the power, 2013-05-24 01:35:31]

Retried try job too often on mac_layout_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=mac_layout...

[commit-bot: I haz the power, 2013-05-24 01:49:50]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexis.menard@intel.com/15912003/4001

[commit-bot: I haz the power, 2013-05-24 03:06:07]

Retried try job too often on mac_layout_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=mac_layout...

[commit-bot: I haz the power, 2013-05-24 08:20:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexis.menard@intel.com/15912003/4001

[commit-bot: I haz the power, 2013-05-24 09:48:59]

Retried try job too often on linux_layout_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_layo...

[commit-bot: I haz the power, 2013-05-24 09:50:15]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/alexis.menard@intel.com/15912003/4001

[commit-bot: I haz the power, 2013-05-24 11:23:29]

Retried try job too often on linux_layout_rel for step(s) webkit_tests
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_layo...

[darktears, 2013-05-24 11:42:14]

Committed patchset #2 manually as r151074.