V8 Buildbot: Speculative fix for guessing msvs version on try server.
V8 Buildbot: Speculative fix for guessing msvs version on try server.

For some reason gyp does not correctly guess the installed msvs version when creating msvs solution files one the v8 try server slaves. This environment variable will give it a hint.

TBR=pschmidt@chromium.org
BUG=

Review URL: https://codereview.chromium.org/115513002

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@240734 0039d316-1c4b-4281-b951-d872f2087c98

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