Replaces the use of data().role with AXNode::GetRole() in ui/accessibility

This patch switches all the code under ui/accessibility to use AXNode::GetRole() for
retrieving the role of a node in the accessibility tree instead of AXNodeData::role.
There are two reasons for this:
1. Retrieving all the other attributes in AXNodeData has already been switched over to
equivalent getters on AXNode, in order to facilitate computing some of those attributes
on the browser.
Retrieving the role by simply accessing its data value would be inconsistent with how other attributes
are now retrieved.
2. It will allow potentially computing the role on the browser via AXComputedNodeData.

R=dmazzoni@chromium.org, aleventhal@chromium.org

AX-Relnotes: n/a.
Change-Id: I1d4127e86c857fb118c3852db75d2ccb93d3f81e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3115096


Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/main@{#915100}