cc: Disable image hijack canvas when we don't have images in the map.
cc: Disable image hijack canvas when we don't have images in the map.

This patch ensures that we don't use the hijack canvas when there are
no images in the discardable map, since this is the way we "disable"
things on android.

Note we already do this in PlaybackToSharedCanvas, so this is just
ensuring that we also do that in PlaybackToCanvas.

BUG=591246
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1756413002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378913}

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