Simplify PositionAlgorithm<Strategy>::containerText() implementation
Simplify PositionAlgorithm<Strategy>::containerText() implementation

This patch changes |PositionAlgorithm<Strategy>::containerText()| simpler
for improving code health.

BUG=n/a
TEST=n/a; no behavior changes

Review URL: https://codereview.chromium.org/1261563004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@199947 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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