Have Document::markers() return a reference

Have Document::markers() return a reference instead of a pointer as it can
never return null. This makes the calling code look safer.

R=adamk

Review URL: https://codereview.chromium.org/181693003

git-svn-id: svn://svn.chromium.org/blink/trunk@167957 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Fix mishandling of initial fill size value

This patch fixes a mistake in CSSToStyleMap::mapFillSize() where
CSSInitialValues were assumed to also be CSSPrimitiveValues.
Instead of setting the initial values it would unset the size entirely.
This change resolves a divide by zero when attempting to interpolate with
elements that have unset background sizes.

BUG=346121

Review URL: https://codereview.chromium.org/176963009

git-svn-id: svn://svn.chromium.org/blink/trunk@167958 bbb929c8-8fbe-4397-9dbb-9b2b20218538