[ContentIndex] Add a `contentdelete` SW event.

Add a SW event that's called for developers to handle users deleting
an entry.

This CL adds the IDL definitions and the blink plumbing required to
dispatch the event.

Bug: 973844
Change-Id: I450e43a9b95f7ea3a10ee5d399db0c321eeeb26d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1702423
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Rayan Kanso <rayankans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679060}
23 files changed