[Fuzz] Init command line before parsing a payment manifest

Before this patch, fuzz test for payment method manifest parser failed
because the command line data structure was not initialized. The command
line was being checked for the flag
--unsafely-treat-insecure-origin-as-secure=<origin> to validate URLs for
payment context.

This patch initializes the command line in the manifest parsing fuzz
test and adds class comments for the classes that require a command
line.

After this patch, fuzz test for payment method manifest does not fail
with the repro case reported in the bug.

Bug: 966694
Change-Id: I3544518468594a6903f713b026fba5bee217e295
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628043
Auto-Submit: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663060}
6 files changed