Linux: Remove second copy of NaCl runtime, used when nacl_helper fails

Before this change, if the NaCl zygote (nacl_helper) failed to start,
NaCl would fall back to launching NaCl loader processes via the
Chromium zygote process, using the second copy of the NaCl runtime
that was still being linked into the "chrome" executable.

There were two problems with this:

 * It bloats the binaries with two copies of NaCl (one in nacl_helper,
   one in chrome).

 * This fallback case was not tested at all on the buildbots, and in
   fact it got broken a while ago.  It would fail with the error
   "AssertNoOtherSignalHandlers: A signal handler is registered for
   signal 17".

Running this untested code path is risky from a security point of
view, so it's better to remove it.

It's enough to remove chrome's reference to NaClMain().  Also,
changing CanHelp() to always return true for nacl-loader processes
stops the zygote from trying the fallback if nacl_helper failed.

Size of out/Release/chrome:
  Before:  145989408
  After:   144569176
This saves 1.4MB.

BUG=118263
TEST=NaCl tests in browser_tests; also tested error from
  PPAPINaClNewlibTest.Core with nacl_helper removed

Review URL: https://codereview.chromium.org/41043002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230891 0039d316-1c4b-4281-b951-d872f2087c98
Really mark SyncTest as disabled

Reverts https://codereview.chromium.org/39933002/ and disables the
test properly.

NOTRY=true
BUG=311091
R=nyquist@chromium.org

Review URL: https://codereview.chromium.org/42103005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230892 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230892