views: Remove the clear to transparent in View::OnPaintLayer
views: Remove the clear to transparent in View::OnPaintLayer

This clear was added when OnPaintLayer was introduced in
https://codereview.chromium.org/7845033, then later only done when the
layer's content was not fully opaque.  However the clear is redundant
with the raster done by the compositor. When the layer's content is
not fully opaque, the compositor will clear the resources it rasters
to transparent before rastering the content on top. This DrawColor
with kClear_Mode will do the same thing again.

R=sky

Review URL: https://codereview.chromium.org/1202663002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#336000}

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