DevTools: introduce WI.Resource.contentSize() and WI.Resource.lastModified()
The patch adds contentSize() and lastModified() methods to the WI.Resource.
The WI.Resource gets the data either from the resource payload or from
the attached request, if any.
BUG=none
Committed:
https://crrev.com/bdb833dea5283a3017b7d291ce984317a4c34905
Cr-Commit-Position: refs/heads/master@{#425508}