[blink] Pass SkBitmap by reference in WebImageLayer::setBitmap

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.
Along with this, it has to do some book keeping with respect to reference counts, updating pixel lock count
etc.

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

R=abarth@chromium.org, eseidel@chromium.org

Review URL: https://codereview.chromium.org/583733002

git-svn-id: svn://svn.chromium.org/blink/trunk@182486 bbb929c8-8fbe-4397-9dbb-9b2b20218538