ImageFrame: Simplify alpha premultiplication calculation
ImageFrame: Simplify alpha premultiplication calculation

Write the computation in-place. There are various forms of this
calculation (see crbug.com/234071) and it can be shown that the
implementation here is bit-exact with SkMulDiv255Trunc(c,a) but
with fewer operations.

No change in behavior, no new tests.

BUG=258324

Review URL: https://chromiumcodereview.appspot.com/18801017

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@153845 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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