Migrate InstallSigner to SimpleURLLoader

Manual test: <out>/chrome --extensions-install-verification="enforce_strict"
TEST=covered by existing tests.

BUG=773295

Change-Id: I116d5c826935c6b5c6c9d062cf55422b7b1915f6
Reviewed-on: https://chromium-review.googlesource.com/1052867
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558130}
3 files changed