Allow early exit from pruneDeadResources() when targetSize is 0.

Pages that use many image elements were spending a lot of time in 
pruneDeadResources when both deadSize and targetSize were 0. Since a deadSize
of 0 means that there are no dead resource to handle, we can exit early in the case
of deadSize == targetSize == 0. 

TEST=improves performance of telemetry test many_images.html by ~40%.
R=junov@chromium.org,senorblanco@chromium.org

Review URL: https://chromiumcodereview.appspot.com/22616003

git-svn-id: svn://svn.chromium.org/blink/trunk@155815 bbb929c8-8fbe-4397-9dbb-9b2b20218538