ozone: Fix flow control regression in ARM swap buffers code path
ozone: Fix flow control regression in ARM swap buffers code path

The early return after PostTaskAndReply() in
GLSurfaceOzoneSurfaceless::SwapBuffersAsync was erroneously removed in
f960a95 ("Split up synchronous & asynchronous swap buffers APIs").  This
adds it back.

It also removes the else after return; that's against the style guide.

BUG=560553
TEST=boot chrome on veyron_jerry

Review URL: https://codereview.chromium.org/1475683002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#361349}

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