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Implements AXLineForTextMarker and corrects the implementation of AXTextMarkerRangeForLine and insertionPointLineNumber

1. NSAccessibilityLineForTextMarker should return the line number within the object
it is called on where the given text marker is located.

2. NSAccessibilityTextMarkerRangeForLineParameterizedAttribute was not implemented correctly.
It should return a text marker range spanning the given line within
the object it is called on.
It doesn't receive a text marker as a parameter but an integer representing the
line number.

3. Per Safari, "insertionPointLineNumber" should return nil when the selection is not collapsed
or when called on an object that does not contain the caret.

4. Took the opportunity to re-write the line calculation logic using C++ algorithms
and switch away from using the deprecated sel_start and sel_end attributes.

R=dmazzoni@chromium.org

Bug: 990931
Change-Id: I96d7aaf56c1603c13f58827bb58b8bf467d91994
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1812100Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Nektarios Paisios <nektar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699334}