Clear Content from Item Map When Removing Placeholder

Problem
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https://github.com/WebFilings/wf-uicomponents/pull/76 introduced a regression in some use cases by inconsistently clearing content when removing placeholders. Item maps (used by the ScrollList in peek and single mode) were not cleared, causing some consumers to have stray, leftover layers.

Solution
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Ensure that when recycling placeholders with item maps, that the content in those item maps is fully cleared, instead of simply removing the content container for the placeholder.

Unit Tests
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Modified existing.

How To +10/QA
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- Travis CI should pass.
- See internal tracking tickets for consuming code higher up the food chain.

@lancefisher-wf @robbecker-wf @patkujawa-wf @georgelesica-wf @tomconnell-wf @todbachman-wf