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I've been playing around with a build of Chrome that asserts all bitmaps
recorded into pictures are immutable.  Most are, but this one popped out
on sites that use native widgets (checkboxes, text fields, etc.).

While we're here, we can just call SkCanvas::setMatrix() instead of
concating the matrix with its inverse.  That works fine with SkPicture now.

BUG=

Review URL: https://codereview.chromium.org/801553002

Cr-Commit-Position: refs/heads/master@{#308088}