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This change updates the headers' aria role to be 'heading' instead of
'group', which is a more appropriate role given the definitions from w3.
This also improves screen readers' understanding of these text blocks.

Bug: 1116623
Change-Id: Iac4fa60c72da8a130bd28841889a3826a755c8e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357461Reviewed-by: dpapad <dpapad@chromium.org>
Commit-Queue: Cliff Smolinsky <cliffsmo@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#798831}