[AW NS] Fix crash in AwDownloadManagerDelegate::InterceptDownloadIfApplicable

The crashes occur (on beta channel) when network service is enabled and
web_contents are null, see crbug.com/965488.

Fix: Similar to chrome, early-out when web_contents is null.

BUG=841556,965488

Change-Id: I37a1f5c4d992507adfc4693f4d348ecba993c387
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1622205
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#662578}