Add variable component for chromium multi dll build.
Add variable component for chromium multi dll build.

Variable component is for cases where we would like to
build some components as shared libraries but still need
to keep the existing variable "library" for static
libraries.

The changes here are basically copied from chromium gyp
changes for the same purpose. For details, see:
http://src.chromium.org/viewvc/chrome?view=rev&revision=50362

R=bradnelson@google.com
BUG=46311
TEST=none

Review URL: http://codereview.chromium.org/2872015

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@2583 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client