Avoid unneeded argument copying in GenericHashTraits

Avoid unneeded argument copying in GenericHashTraits 'peek' and 'passOut'
methods.
The problem was that those methods accepted arguments by const reference
but returned them by value creating extra temporary copies.
This patch makes these methods return the arguments by const reference too
(no temporaries are created), a little decrease of content_shell binary
size is observed: 90110792 - 90106416 = 4376 bytes (Linux x64, GCC 4.6.3,
release static library  build).

Review URL: https://codereview.chromium.org/189513014

git-svn-id: svn://svn.chromium.org/blink/trunk@168994 bbb929c8-8fbe-4397-9dbb-9b2b20218538