fix(runtime-core): look up prop value from props.

[underfin]

fix #1236

It fixed in 3.0.0-beta.15 development build. But it doesn't work in 3.0.0-beta.15 production build.

###Bug Description
In dev can be look up successful, because expose props into ctx by exposePropsOnRenderContext.
Other, it will not exist type.props when extend props from mixin.
So, we need look up the prop values from props.

[yyx990803]

instance.props is not the source of truth since if an absent prop won't be set on it. We still need to use the normalized props options - the reason of the bug was we are only checking it when type.props is present - but when a component has props from mixins but no props declaration on itself it would be skipped.

See e0d19a6