Move base BUILD.gn files to main tree.

This also removes thread_local_storage.cc which was removed from the .gyp file.

R=viettrungluu@chromium.org
TBR=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/119323003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242810 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242810
gn: Mark the generator rule with `generator = 1`.

According to http://martine.github.io/ninja/manual.html#ref_rule , setting
`generator` has the effect of not rerunning ninja if the generator commandline
changes. Without this, the generator will rerun on the first build, after it
has just run.

BUG=331234
TEST=Follow tutorial in clean checkout. Run `ninja -C out/Default hello_world`.
Doesn't try to run gn as first step.

Review URL: https://codereview.chromium.org/105303004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242811 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242811