appcache: force enable appcache on renderer side of webview

This patch supported WebView for the browser-side:
https://chromium-review.googlesource.com/c/chromium/src/+/2190220

However, there was nothing to tell the renderer to also enable AppCache
like ChromeContentBrowserClient::AppendExtraCommandLineSwitches does.
This patch adds that same code for AwContentBrowserClient.

This fixes webview tests when kAppCacheRequireOriginTrial is enabled.

Bug: 582750
Change-Id: Iadc572e8a825225ffc4879ffedbd1bf14e8115af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2233709
Commit-Queue: enne <enne@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Auto-Submit: enne <enne@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#775825}