[SRU] webbrowser-app bug fixes

[Impact]

This SRU groups 9 separate bug fixes that went into bug:webbrowser-app since xenial was released. Those were carefully picked because of their impact on desktop users of webbrowser-app and webapp-container (images for phones and tablets are built with the stable-phone-overlay PPA so they don’t require backports):

 - bug #1565055: support for google hangouts (manual test case in bug description)
 - bug #1573017: SAML detection logic broken in webapp container (changeset has autopilot tests)
 - bug #1572673: invalid variable access error in webapp container (changeset has autopilot tests)
 - bug #1466427: dynamic Ubuntu version in default UA string (manual test case in bug description)
 - bug #1576639: fine-tune the custom memory-pressure handler (manual test case in bug description)
 - bug #1581025: secure connection icon not showing (manual test case in bug description)
 - bug #1580290: only load the current tab when the session is restored (manual test case in bug description)
 - bug #1577806: browser and container open under the same instance (changeset has autopilot tests + manual test case in bug description)
 - bug #1565063: dynamic Chromium version in default UA string (manual test case in bug description)

[Test Case]

Each individual bug report has details of how to reproduce the bug and how to verify it is fixed, along with additional unit/autopilot tests where possible/applicable.
All unit tests are being run at package build time.
To run autopilot tests, install the {webbrowser-app,webapp-container}-autopilot packages, and run the following commands:

  autopilot3 run webbrowser_app
  autopilot3 run webapp_container

Beware that this is time-consuming and that it takes over user input (keyboard and mouse), so better to run those in a spare machine on the side.
All webapp_container tests should pass.
Of the 184 tests in the webbrowser_app test suite, 3 are known to fail. Those are test issues, not actual functional issues with the app, so they can be safely ignored:
  test_geolocation.TestGeolocation.test_geolocation_accept
  test_geolocation.TestGeolocation.test_geolocation_deny
  test_new_tab_view.TestNewTabViewContentsWide.test_drag_bookmarks

[Regression Potential]

General UX regressions should be caught by existing autopilot tests (both apps have quite an extensive test suite). The most intrusive change in this SRU is also the most critical one, it’s the fix for bug #1577806. As such, it has been particularly carefully tested.