Add accessibility extra and clarification callbacks to custom tabs.

During launch review we got feedback that the button should always
have an accessibility string. Added it to the builder as a must have
key as well. Clarified button related specs.

Also added two more callback signals to remedy cases where we get two
pageLoadStarted signals back to back due to user action or internal
failures interrrupting page load.

b/23079160

Change-Id: I7f590d1df6e3ac9757e70e2518363d038a5aaef5
3 files changed