New object attributes to disambiguate description sources

NVDA screen reader team has asked for some changes to how descriptions
are exposed in IAccessible2 so that they don't have to automatically
announce tooltips, while still announcing other annotations.

New object attributes:
* "description" now reflects aria-description, but not other description
  sources. This matches what Firefox does now.
* "description-from" has one one the following values:
  - "aria-description"
  - "aria-describedby"
  - "ruby-annotation"
  - "summary"
  - "table-caption"
  - "tooltip" (either via @title or aria-describedby + role="tooltip")
  - "button-label" (when the name was from something else)

The above may require future tweaks but has been agreed to in principle
with NVDA and Mozilla.

Bug: 1194991
Change-Id: Ie20486e569b6f892e85c143119d1eea4d91efaf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2799998


Commit-Queue: Aaron Leventhal <aleventhal@chromium.org>
Auto-Submit: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#871966}