Improve GN value origins for error messages.

Improve GN value origins for error messages.

Sets the origin for a value when the value is dereferenced. Previously if you did
  foo = 6
  deps = [ foo ]
You would get the message:
  ERROR at //BUILD.gn:15:9: This is not a string.
    foo = 6
          ^
Which in this case is pretty confusing since that's obviously the case, and you really want to know where it was dereferenced.

With this patch the message is:
  ERROR at //BUILD.gn:21:5: This is not a string.
      foo,
      ^--
  Instead I see a integer = 6
which is more actionable.

R=ajwong@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=288131

[awong, 2014-08-07 19:03:55]

LGTM

Nice!

[brettw, 2014-08-07 20:11:07]

Committed patchset #2 manually as 288131 (presubmit successful).