Don't modify the PWD in gyp_v8

Don't modify the PWD in gyp_v8

When running gyp in parallel mode (soon the default), we need to be able
to find the script by looking at the path relative to the PWD when the
script was originally invoked.

Instead of changing the directory and relying on relative paths, just
use absolute paths (as gyp_chromium does).

BUG=v8:2973
R=ulan@chromium.org
LOG=n

Committed: https://code.google.com/p/v8/source/detail?r=18439

[ulan, 2014-01-02 13:49:12]

lgtm

[jochen (gone - plz use gerrit), 2014-01-02 13:51:00]

Committed patchset #1 manually as r18439 (presubmit successful).