WebappActivityTestRule#startWebappActivity() is currently hard to understand

In particular:
- It is unclear what WebappActivityTestRule#waitUntilIdle() waits for. Does it return before or after Tab#loadUrl() is called? (I did some local testing and have confirmed that it returns prior to Tab#loadUrl() is called at least some of the time)
- If WebappActivityTestRule#waitUntilIdle() returns after the page has finished loading, WebappActivityTestRule#waitUntilIdle() is also called after the splash screen hide animation has started. It is possible that the splash screen is fully hidden when WebappActivityTestRule#waitUntilIdle() in which case mActivityTestRule#waitUntilSplashScreenAppears() would fail

I suggest having two methods in WebappActivityTestRule:
startWebappActivity() which will return when about:blank has finished loading
startWebappActivityWithSlowLoadingPage() which will return once the Tab has been created. You can use EmbeddedTestServer.java with the special URL "/slow?1967" to get the URL to a page which takes a long time - 1967 seconds to load

It would be nice if we can get WebappActivityTestRule#startWebappActivity(), WebappActivityTestRule#startWebappActivityWithSlowLoadingPage(), WebApkIntegrationTest#WebApkIntegrationTest(), and MultiActivityTestComment#waitForFullLoad() to share as much code as possible