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This change is an optimization of AsTextPosition().

It's part of the larger performance improvement related to the expensive
calls to MaxTextOffset(). AXposition::CreatePositionAtStartOfDocument()
calls repetitively AxTextPosition() to allow converting from a tree
position and to a text position and back, but this required calling
MaxTextOffsetInParent() a lot.

In a simple navigate-by-word situation, every call to AsTextPosition()
would call MaxTextOffsetInParent uselessly. This change fixes it.

There is no need to get the MaxTextOffset of an AXPositionInstance when
the |text_offset_| will be positioned at the beginning of the anchor.
We expect to position the text_offset_ of a Text Position at the
beginning of the anchor when |text_offset_| is either invalid, already
at 0 or less than the actual new_offset. All I did was adding a check
for these conditions before calling MaxTextOffsetInParent().

I measured the perf difference with a trace captured with WPR and
analyzed with WPA. In both cases, the traces consist of 10 words read
by Narrator with navigate-by-word navigation.

Without patch:
  ExpandToEnclosingUnit: 2,169 ms
  CreatePositionAtStartOfDocument: 103 ms

With patch:
  ExpandToEnclosingUnit: 1,851 ms
  CreatePositionAtStartOfDocument: <1 ms

Bug: 928948
Change-Id: I41d896dd25519eaba6a8c82d0ffacd3fbdedd909
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1811221
Commit-Queue: Benjamin Beaudry <benjamin.beaudry@microsoft.com>
Reviewed-by: Kurt Catti-Schmidt <kschmi@microsoft.com>
Reviewed-by: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699478}