Clean up the environment MB invokes GYP with.

Previously MB would take the environment from the
calling (parent process), update GYP_DEFINES as needed,
and then invoke gyp_chromium. This had the unfortunate effect
that if the parent environment had GYP_CHROMIUM_NO_ACTION
set, gyp_chromium would bail out without doing anything.

This patch re-organizes the logic so that MB will inherit
the parent environment but clear out any GYP-related variables,
and then only set the things it explicitly wants to be set.
Further, the environment will now only be manipulated in one,
so it'll be easier to understand what's going on and keep
things consistent.

R=scottmg@chromium.org
BUG=530733

Review URL: https://codereview.chromium.org/1378003002

Cr-Commit-Position: refs/heads/master@{#351440}

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