Allow chromium url to be overwritten as a custom variable in DEPS.

Using this work around, buildbot can set the chromium_url variable to the golo mirror.

BUG=372738
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/281053003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21296 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
Revert PropertyCallbackInfo::This() signature change from r21022.

This exposed an internal implementation detail, which could be handled
differently.

BUG=v8:3274
LOG=y
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/285643008

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21297 ce2b1a6d-e550-0410-aec6-3dcde31c8c00