Loading: move TextResource to core/loader/resource
To remove core/ dependency so that core/fetch can be moved under
platform, move TextResource and TextResourceClient classes
to core/loader/resource/.
BUG=
655920
Committed:
https://crrev.com/87a093df53d87549a8b1d54850b15af9a477bcc5
Cr-Commit-Position: refs/heads/master@{#435885}