[blink] Pass SkBitmap by reference in WebNinePatchLayer::setBitmap and refactor

Although passing SkBitmap by value doesn't copy the underlying pixels, it indeed
involves memcpy (~64 bytes to be precise) to replicate the settings from the
other bitmap in the copy constructor.

This overhead, albeit minor, can be avoided by passing SkBitmap as reference.

R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/594193002

git-svn-id: svn://svn.chromium.org/blink/trunk@182559 bbb929c8-8fbe-4397-9dbb-9b2b20218538