Upstream: Initialize ChromeWebApkHost in WebappLaunchActivity.

This CL fixes a bug that when a WebAPK is launched without Chrome
is running, WebApkValidator returns false for a valid WebAPK since
the WebApkValidator hasn't been initialized.

BUG=609122

Review-Url: https://codereview.chromium.org/2056383002
Cr-Commit-Position: refs/heads/master@{#399544}
2 files changed