[Tools] Fixed xwalk_android_gyp issue

according to the chromium upstream patch d9584323 and 50f36a01,
the comunity decided to stop exporting CHROME_SRC and
ENVSETUP_GYP_CHROME_SRC out of build/android/envsetup.sh, which
will break xwalk_android_gyp.
So we need to get rid of the dependency on those exported variables
in our own script.