NaCl: Move all browser tests into nacl_irt_test_env

NaCl: Move all browser tests into nacl_irt_test_env

IRT-using nexes are normally built in nacl_irt_test_env, not nacl_env.
All of chrome_browser_tests uses the IRT, so changing these tests to
use nacl_irt_test_env brings chrome_browser_tests into line with the
rest of the NaCl Scons build.

This will let us fix libimc_syscalls to call GC hooks without breaking
linking the nexes in chrome_browser_tests (since some tests there use
libimc_syscalls).

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3253
TEST=nacl_integration + count tests to check we haven't lost any

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=176224

[Mark Seaborn, 2013-01-10 20:21:12]

This depends on rolling https://codereview.chromium.org/11820043 into Chromium.

[Nick Bray (chromium), 2013-01-10 22:24:57]

LGTM