Why: Simpler build code. If everybody includes it, it should be included automatically.
Why now: The webkit chromium builds need it be specified, since can't default to build/common.gypi.

What was done:
1. build/common.gypi's contents were moved to a new file build/gyp_chromium.gypi
2. tools/gyp/gyp_chromium was moved to build/gyp_chromium and made to automatically include build/gyp_chromium.gypi.
3. lots of gyp files were fixed to not refer to build/common.gypi any more.
4. o3d which also builds independently of chrome, was fixed to have a gyp_o3d that includes gyp_chromium.gypi too.
5. build/common.gypi was left empty, because there are some external projects that still refer to it.

Things that are left to do after this patch is in:
1. The following external files (in other repositories) need to stop include common.gypi
  ./third_party/hunspell/hunspell.gyp
  ./third_party/icu/icu.gyp
  ./v8/tools/gyp/v8.gyp
2. Once nobody refers to common.gypi anymore, delete common.gypi
   -or-
   Delete gyp_chromium.gypi and move its content back to common.gypi

Tested on mac, win and linux. On win, got a few unit tests errors on chrome bookmarks, which should not be related. I'm running again with clobber to verify.

Review URL: http://codereview.chromium.org/206006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@26302 0039d316-1c4b-4281-b951-d872f2087c98