Start stitching GYP-generated files into the Linux build:

* Add a GYP=1 command-line variable to use the gyp-generated files
  (which are generated side-by-side until everything's okay enough
  to cut over for real).

* Rearrange existing *.scons files to match the layout of the
  gyp-generated ones, so the transition will be easier:
  * base.scons (the wrapping logic that calls the other *.scons files)
    => base_sln.scons
  * base_lib.scons (the library itself)
    => base.scons (matching the gyp target generation)
  * gfx/base_gfx.scons
    => base_gfx.scons (with necessary prepending of "gfx/" to path names)

build/SConscript.main fixes:
* Use an internal ${_GYP} infix variable to select the right flavor
  of *.scons file (multiple places)
* When building with GYP=1, only load the one component *_sln_gyp.scons
  file, because gyp has already created it with knowledge of all the
  right dependent *_gyp.scons files to load.

Linux gyp build fixes:
* Add -32 to $ASFLAGS for generating a 32-bit libicudata.a from the
  now-checked in .s.
* Add -Wno-unused and -Wno-unused-function to skia.
Review URL: http://codereview.chromium.org/28207

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@10759 0039d316-1c4b-4281-b951-d872f2087c98