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This patch fixes logic for snapping layers to physical pixel boundaries,
when pixel canvas is not enabled, by allowing the subpixel offset to be
dynamically computed. Previously, layers could get stuck at 100% DPI
pixel boundaries (making some UI blurry on platforms where fractional
scale factors are used, e.g. Windows), as once an explicit value was
set, the offset would no longer be dynamically computed.

This patch
 - Updates SnapLayerToPixelBoundary to stop View from setting explicit
   subpixel offsets for non-pixel-canvas platforms so that they may be
   auto computed.
 - Updates view_unittest.

Bug: 992730
Change-Id: I8e8c87107f13157e92f125bcfb281226ebc6c422
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761301Reviewed-by: Malay Keshav <malaykeshav@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Brian Liu Xu <brx@microsoft.com>
Auto-Submit: Brian Liu Xu <brx@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#689171}