Engine: Add loadData() functionality

[pocmo]

Currently we can just load URLs. However WebView and GeckoView also have the ability to load HTML directly. That's something we use in our apps to display internal pages or custom error pages.

That's the API in the Focus abstraction layer:
https://github.com/mozilla-mobile/focus-android/blob/master/app/src/main/java/org/mozilla/focus/web/IWebView.java#L143

Let's compare GeckoView and WebView and try to find a good API that works for both.

┆Issue is synchronized with this Jira Task

[jonalmeida]

Comparing all the loadData APIs that we currently have:

// GeckoView
public void loadData(final byte[] bytes, final String mimeType, final String baseUri)

// WebView
public void loadDataWithBaseURL(String baseUrl, String data, String mimeType, String encoding, String historyUrl)

// IWebView
public void loadData(String baseURL, String data, String mimeType, String encoding, String historyURL)

So it looks like the IWebView interface is identical to the WebView one.

After some investigating, it doesn't look like GeckoView has support for a custom historyUrl (a page to default to when pressed back a page url to show in the history tab). WebView defaults this to about:blank when null which we can do as well (or about:home?).

For encoding, even though WebView defaults to ASCII we should default to UTF-8 since that is Android's default and I don't see a way to change this for GeckoView.

GeckoView also has loadData and loadString where the latter just base64 encodes the string and calls loadData.

I recommend we stick with:

public void loadData(@NonNull final String data, @Nullable final String mimeType, @Nullable final String baseUri)

For GeckoView, this will call loadString instead. mimeType will default to 'text/html'.

[jonalmeida]

Default encoding for Android is UTF-8: https://developer.android.com/reference/java/nio/charset/Charset#defaultCharset()

Focus always calls loadData with the historyUrl as the same value as baseUri. So far, this has only been used for about:home and about:blank:

public static final String URL_ABOUT = "focus:about";

webView.loadData(URL_ABOUT, data, "text/html", "UTF-8", URL_ABOUT);

Fennec however doesn't add anything to history at all when loadData is called (from testing in Nightly).

Created bug 1483677 to track adding this to GeckoView, but for now we don't need to add it. Tabled this since Gecko doesn't support baseUrl any more.