Dedupe files for goma parallel build.
Dedupe files for goma parallel build.

build_nexe.py automatically starts parallel build if goma is available.
If there are duplicated files listed to build, parallel build may cause
race condition.  Two threads may try to remove the same file, and it causes
the unexpected OSError.

To avoid this situation, files are deduped using set().  Since building
the same file twice is also waste of resources in non-parallel build,
files are deduped early in build_nexe.py.

Landing with NOTRY as on of the trybots is currently
failing consistently.

BUG= https://code.google.com/p/chromium/issues/detail?id=254599
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/18152003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@11748 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client