Fix use of LOAD= with WantSystemLib()...

Fix use of LOAD= with WantSystemLib() (we could blow up if a variable
hadn't been added to the config) and extend use of LOAD= into submodules:
* Add a ChromeLoadSConscriptModules() method that encapsulates the
  conditional logic, and makes things more readable by specifying
  component names as keyword arguments, not hard-coding the logic
  as a series of if-tests.
* Put the ChromeLoadSConscriptModules() logic in a Tool module in
  site_scons/site_tools, so it doesn't clutter up
  build/SConscript.main directly.
* Move env.WantSystemLib() calls into the individual *.scons files,
  so we call them each time (or not, based one LOAD=) and the config
  itself just returns if the system library is requested and we
  don't need to build anything locally.
* Move the settings where a library name changes based on whether or
  not the system lib is being used into the using_*.scons files,
  so they're available to clients independently of whether or not the
  component's *.scons configuration is loaded.
* While here:  rename the affected third_party SConscript files:
  third_party/libjpeg/SConscript => third_party/libjpeg/libjpeg.scons
  third_party/libxml/SConscript => third_party/libxml/libxml.scons
  third_party/libxslt/SConscript => third_party/libxslt/libxslt.scons
* While here:  move the Chrome{Program,SharedLibrary}() etc. builder
  definitions from build/SConscript.main to a new too
  Ad the ChromeLoadSConscriptModules() logic in a Tool module,
  to remove more clutter from build/SConscript.main.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=5820

[sgk, 2008-11-20 20:39:01]

Evan:  I ran into this quirk in how WantSystemLib() depended on variables like
XML_LIB, and was mostly done with this LOAD= stuff anyway, so decided to forge
ahead and polish off the CL.

[bradn, 2008-11-20 20:44:37]

LGTM

[Evan Martin, 2008-11-20 21:11:42]

random style

http://codereview.chromium.org/11430/diff/210/18
File build/SConscript.v8 (right):

http://codereview.chromium.org/11430/diff/210/18#newcode10
Line 10: Return()
four spaces :\

http://codereview.chromium.org/11430/diff/210/29
File chrome/chrome.scons (right):

http://codereview.chromium.org/11430/diff/210/29#newcode56
Line 56: activex_test_controls =
'test/activex_test_control/activex_test_control.scons',
80 columns :\

http://codereview.chromium.org/11430/diff/210/27
File site_scons/site_tools/chromium_builders.py (right):

http://codereview.chromium.org/11430/diff/210/27#newcode12
Line 12: def ChromeProgram(env, *args, **kw):
two-space tabs :\

[sgk, 2008-11-21 01:06:28]

http://codereview.chromium.org/11430/diff/210/18
File build/SConscript.v8 (right):

http://codereview.chromium.org/11430/diff/210/18#newcode10
Line 10: Return()
On 2008/11/20 21:11:42, Evan Martin wrote:
> four spaces :\

Thanks for the catch; done.  (Also in other uses.)

http://codereview.chromium.org/11430/diff/210/29
File chrome/chrome.scons (right):

http://codereview.chromium.org/11430/diff/210/29#newcode56
Line 56: activex_test_controls =
'test/activex_test_control/activex_test_control.scons',
On 2008/11/20 21:11:42, Evan Martin wrote:
> 80 columns :\

Done.

http://codereview.chromium.org/11430/diff/210/27
File site_scons/site_tools/chromium_builders.py (right):

http://codereview.chromium.org/11430/diff/210/27#newcode12
Line 12: def ChromeProgram(env, *args, **kw):
On 2008/11/20 21:11:42, Evan Martin wrote:
> two-space tabs :\

Done; also in other chrome*py module.