Add comments explaining why `SystemEngineView.initWebView` can create WebViews; some duplication

[mcomella]

In the main code path, a NestedWebView is created for each SystemEngineSession via its WebView variable:

android-components/components/browser/engine-system/src/main/java/mozilla/components/browser/engine/system/SystemEngineSession.kt

Line 55 in a58f4da

@Volatile var webView: WebView = NestedWebView(context)

This is passed from the session to the engineView in SystemEngineView.render:

android-components/components/browser/engine-system/src/main/java/mozilla/components/browser/engine/system/SystemEngineView.kt

Line 77 in a58f4da

addView(initWebView(session.webView))

However, sometimes this can be null apparently – NestedWebView can also created as a default argument in SystemEngineView.initWebView:

android-components/components/browser/engine-system/src/main/java/mozilla/components/browser/engine/system/SystemEngineView.kt

Line 105 in a58f4da

internal fun initWebView(webView: WebView = NestedWebView(context)): WebView {

There are two problems here:

• There are no comments explaining why initWebView's default argument would need to be used (e.g. I don't know why)
• The NestedWebView construction call is duplicated in two places (though init is shared)

I'm more concerned about the lack of comments than the duplicated constructor because the init is shared.

┆Issue is synchronized with this Jira Task

[csadilek]

Just looked into this. We can actually remove the default argument now. It's a leftover.