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In the layout tree, a LayoutText corresponds to a static text node in
HTML, which can span more than one line. Each line of text is represented
by an InlineTextBox.

In the accessibility tree, we have an AXLayoutObject for the LayoutText,
and an AXInlineTextBox for the InlineTextBox.

The coordinates of the AXInlineTextBox need to be expressed relative to
its parent object, just like everything in the accessibility tree. It turns
out that we need the local bounding box of the parent object (the LayoutText)
to compute this.

However, this leads to an inefficiency, because when you have a very large
LayoutText (as an example, a TEXTAREA element with thousands of lines of
text), each AXInlineTextBox keeps querying its parent LayoutText for its
bounds, but the LayoutText iterates over all of its InlineTextBox children
to compute its bounds! This leads to n^2 complexity where n is the number
of lines in the LayoutText.

The solution is just to cache the bounds of the LayoutText, so it only
needs to be computed once.

Bug: 875344
Change-Id: I97578ec0eadc597f962b142fb074e6929e834802
Reviewed-on: https://chromium-review.googlesource.com/1180647Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584528}