DevTools: show layers in Timeline frame details

This replaces frame statistics with Layers3DView if layer tree is available
for a given frame.

BUG=

Review URL: https://codereview.chromium.org/293063006

git-svn-id: svn://svn.chromium.org/blink/trunk@174605 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Cache whether we've already generated symbols

Trying to regenerate them if the binary wasn't updated is relatively
cheap, but we shouldn't need to call the external script if we don't
have to.

BUG=none
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/292333002

git-svn-id: svn://svn.chromium.org/blink/trunk@174606 bbb929c8-8fbe-4397-9dbb-9b2b20218538