Store CPU profiles in temp file on front-end

Serialized CPU profile is saved into a temporary file immediately after it has been received from back-end. This temp file is used later if user selects "save to file", in which case contents of the temp file is copied into the specified destination.

BUG=327298
R=loislo@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/111543002

git-svn-id: svn://svn.chromium.org/blink/trunk@163535 bbb929c8-8fbe-4397-9dbb-9b2b20218538