Add a context menu to the aura/views content shell

This also removes the "Debug" menu, because the functionality moved
to the context menu.

I added a function to Shell and ShellDevtoolsFrontend to inspect
a specific element when opening devtools.

This effectively reverts most of https://codereview.chromium.org/151823002/

R=pfeldman@chromium.org
BUG=

Review URL: https://codereview.chromium.org/140333009

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249566 0039d316-1c4b-4281-b951-d872f2087c98
Do not force scrollbar update if parameters didn't change.

BUG=341145
NOTRY=true

Review URL: https://codereview.chromium.org/138013006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249567 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249566
SVN changes up to revision 249567
Add unit test target for shell_dialogs module.

This is necessary to avoid a circular dependency that on Mac is forbidden:

ui_unittests -> shell_dialogs -> aura -> ui_unittests

The down side is that momentarily we lose automatic test coverage on the bots for the shell_dialogs.
This can be fixed by teaching the buildbot scripts to run it after this lands.

BUG=331669, 35878, 299841
TEST=shell_dialogs_unittests
R=ben@chromium.org

Review URL: https://codereview.chromium.org/151253004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@249568 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 249568