Initialize Canvas on non-Cairo platforms.
Initialize Canvas on non-Cairo platforms.

skia::PlatformCanvas instances are not initialized to 0 on non-Cairo platforms and we should initialize it manually in Canvas.

BUG=NONE

Review URL: https://codereview.chromium.org/1413093003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#355380}

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