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This is for simplifying https://codereview.chromium.org/1010183002/

This also removes 'if (key == kProgramKey) ...' check from
JsonManifest::ResolveKey. This is safe because kProgram key was
never passed to the function before the change.

BUG=nativeclient:3802
TEST=git cl try, manually checked that both ARC and PNaCl demo still work

Review URL: https://codereview.chromium.org/1070233007

Cr-Commit-Position: refs/heads/master@{#325547}