Remove //chrome deps from ExtensionDownloader

This frees ExtensionDownloader of any remaining
dependencies on //chrome in preparation for its
move to //extensions/browser.

The move is left for a follow-up CL.

BUG=398671

Review URL: https://codereview.chromium.org/651403003

Cr-Commit-Position: refs/heads/master@{#299776}
8 files changed