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

Review URL: https://codereview.chromium.org/15912003

git-svn-id: svn://svn.chromium.org/blink/trunk@151074 bbb929c8-8fbe-4397-9dbb-9b2b20218538
3 files changed