compositor: Disable partial raster by default.
compositor: Disable partial raster by default.

After enabling it, BUG=594516 is regressed. Before fixing it, disable it by
default.

BUG=492754, 594516

Review URL: https://codereview.chromium.org/1801093003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#381316}

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