v8_external_snapshot target visible only when using v8_use_external_startup_data

Prevent snapshot_blob.bin from being unnecessarily generated when building all targets.
When such file exists, chrome_tests assumes we will load V8's initial snapshot from
external files, even when such feature is turned off, which prevents the tests from
being successfully compiled.

BUG=421063
LOG=N
R=rmcilroy@chromium.org

Review URL: https://codereview.chromium.org/643163002

Patch from André Baixo <baixo@google.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24550 ce2b1a6d-e550-0410-aec6-3dcde31c8c00