webview: enable file: URLs in UI tests explicitly.

Apps that target R or later default to setAllowFileAccess(false). Update
the WebView UI test code to explicitly enable it before trying to load
file:// URLs in the tests, so that it won't break once we build against
the R SDK.

Bug: 1106471
Change-Id: I863ffefcf2eec947062e2e99f2e40e60ec452fbf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316400Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791826}

android_webview/tools/automated_ui_tests/javatests/src/org/chromium/webview_ui_test/test/util/WebViewSyncWrapper.java

@@ -145,6 +145,7 @@ public class WebViewSyncWrapper {
         runOnUiThread(new Runnable() {
             @Override
             public void run() {
+                mWebView.getSettings().setAllowFileAccess(true);
                 mWebView.loadUrl(FILE_URL_BASE + html);
             }
         });