Web Animations CSS: Fix crash when transitioning background or mask properties

This patch ensures the first layer for background and mask declares that
their values are set. This has no visible effect on the way fill layers
work, this just ensures an assertion made by the Web Animations engine
about fill layer properties is correct.

The current animation implementation did not make use of the flags on a fill layer
that indicate whether a value was set or not and will be agnostic to this change.
In order to implement correct interpolation behaviour[1] for many fill layer
properties these flags need to be used.
The existing FillLayer implementation was not setting these flags correctly for
the initial values, this patch fixes that.

[1] http://www.w3.org/TR/css3-transitions/#animtype-repeatable-list

Fixes crashing tests:
virtual/web-animations-css/transitions/multiple-background-size-transitions.html
virtual/web-animations-css/transitions/shorthand-transitions.html

BUG=257591

Review URL: https://codereview.chromium.org/38573005

git-svn-id: svn://svn.chromium.org/blink/trunk@161438 bbb929c8-8fbe-4397-9dbb-9b2b20218538