Fix main_frame for target _blank links

When opening a target _blank link to a page with multiple iframes
the main_frame tracking code got it wrong and picked one of the
iframes as the main_frame. In this case Runtime.executionContextCreated
gets tricked last for the root frame.

Author: betelgeuse

lib/ferrum/page/frames.rb

@@ -75,8 +75,11 @@ def frames_subscribe
           frame_id = params.dig("context", "auxData", "frameId")
 
           unless @main_frame.id
-            @main_frame.id = frame_id
-            @frames[frame_id] = @main_frame
+            root_frame = command('Page.getFrameTree').dig('frameTree', 'frame', 'id')
+            if frame_id == root_frame
+              @main_frame.id = frame_id
+              @frames[frame_id] = @main_frame
+            end
           end
 
           frame = @frames[frame_id] || Frame.new(frame_id, self)

spec/frame_spec.rb

@@ -27,6 +27,14 @@ module Ferrum
         expect(frame.url).to end_with("/ferrum/get_cookie")
       end
 
+      it 'finds main frame properly' do
+        browser.goto("/ferrum/popup_frames")
+        browser.at_xpath("//a[text()='pop up']").click
+        expect(browser.pages.size).to eq(2)
+        opened_page = browser.pages.last
+        expect(opened_page.main_frame.url).to end_with("/frames")
+      end
+
       it "waits for the frame to load" do
         browser.goto
         browser.execute <<-JS

spec/support/views/popup_frames.erb

@@ -0,0 +1 @@
+<a href="frames" target="_blank">pop up</a>