Move the main entry point *.scons files in the key remaining components...

Move the main entry point *.scons files in the key remaining components
(net, webkit, chrome) to *_main.scons, so we can use GYP=1 on the command
line to switch between old and new while tracking down the remaining
inconsistencies.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=10882

[sgk, 2009-03-04 00:38:12]

[This is a simplest-thing-that-can-work change to make it easier for others to
work in parallel on gyp Linux port issues in the different components.  All of
the other existing SCons files stay where they are, only the entry point changes
to match the generated *_main_gyp.scons files.]

[tony, 2009-03-04 01:46:02]

LGTM