feat: Add Frame#parent method (#523)

Facilitates easier traversal of nested iframes.

Author: myabc

lib/ferrum/frame.rb

@@ -94,6 +94,23 @@ def main?
       @parent_id.nil?
     end
 
+    #
+    # Returns the parent frame if this frame is nested in another one.
+    #
+    # @return [Frame, nil]
+    #
+    # @example
+    #   browser.go_to("https://developer.mozilla.org/en-US/docs/Web/HTML/Element/iframe")
+    #   frame = browser.frames.last
+    #   frame.url # => "https://www.openstreetmap.org/export/embed.html?bbox=-0.004017949104309083%2C51.47612752641776%2C0.00030577182769775396%2C51.478569861898606&layer=mapnik"
+    #   frame.parent.main? # => false
+    #   frame.parent.parent.main? # => false
+    #   frame.parent.parent.parent.main? # => true
+    #
+    def parent
+      @page.frame_by(id: @parent_id) if @parent_id
+    end
+
     #
     # Sets a content of a given frame.
     #

spec/frame_spec.rb

@@ -43,6 +43,12 @@
     expect(opened_page.main_frame.url).to end_with("/frames")
   end
 
+  it "finds parent frame properly" do
+    page.go_to("/frames")
+    parent_frame = page.at_xpath("//iframe").frame.parent
+    expect(parent_frame.url).to end_with("/frames")
+  end
+
   it "waits for the frame to load" do
     page.go_to
     page.execute <<-JS