ARIA submenus violation: Submenus should indicate they're submenus
ARIA submenus violation: Submenus should indicate they're submenus

Soft menus don't announce that they have submenus, so they might
a sight-impaired user who relies on a screen reader may not realize
that the item has an available submenu. This change conforms to
WAI-ARIA guidelines as shown on this sample:

https://www.w3.org/TR/wai-aria-practices-1.1/examples/menubar/menubar-1/menubar-1.html

Bug: 963183
Change-Id: Ic3ad8c509fff993d847f94862c042d5ec04d2e2a
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2213804
Reviewed-by: Brian Cui <eebe54f29fdfbfde69930c5ed5d840454db62ce7@microsoft.com>
Commit-Queue: Robert Paveza <7787129de3d0cb4911c68d958db3023863584560@microsoft.com>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend