compile.py: test build.ninja.orig exists before remove it
compile.py: test build.ninja.orig exists before remove it

TBR=maruel@chromium.org
Review URL: https://codereview.chromium.org/11275032

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@164027 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build