Use constexpr TaskTraits constructor in android_webview.

Use a constexpr constructor that accepts traits in any order instead of
the builder pattern to construct TaskTraits in android_webview.

BUG=713683
R=gab@chromium.org
TBR=tobiasjs@chromium.org

Review-Url: https://codereview.chromium.org/2856203005
Cr-Commit-Position: refs/heads/master@{#469114}