pdf-viewer/mandoline: Fix a crash during teardown.
pdf-viewer/mandoline: Fix a crash during teardown.

Explicitly destroy the root-views during destruction.

BUG=none

Review URL: https://codereview.chromium.org/1393903004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354777}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk