Define PageSource (bazelbuild#198)

PageSource returns the source of the current browsing context active document.

go/launcher/webdriver/webdriver.go

@@ -87,6 +87,8 @@ type WebDriver interface {
 	W3C() bool
 	// CurrentURL returns the URL that the current browser window is looking at.
 	CurrentURL(context.Context) (*url.URL, error)
+	// PageSource returns the source of the current browsing context active document.
+	PageSource(context.Context) (string, error)
 	// NavigateTo navigates the controlled browser to the specified URL.
 	NavigateTo(context.Context, *url.URL) error
 }
@@ -362,6 +364,16 @@ func (d *webDriver) CurrentURL(ctx context.Context) (*url.URL, error) {
 	return current, nil
 }
 
+// PageSource returns the source of the current browsing context active document.
+func (d *webDriver) PageSource(ctx context.Context) (string, error) {
+	var result string
+
+	if err := d.get(ctx, "source", &result); err != nil {
+		return "", err
+	}
+	return result, nil
+}
+
 // NavigateTo navigates the controlled browser to the specified URL.
 func (d *webDriver) NavigateTo(ctx context.Context, u *url.URL) error {
 	return d.post(ctx, "url", map[string]interface{}{