Rename libplatform.a to libblink_platform.a

The old name caused a name conflict with NaCl's libplatform.a in the Xcode
build.

TBR=jamesr
BUG=297477

Review URL: https://codereview.chromium.org/24505003

git-svn-id: svn://svn.chromium.org/blink/trunk@158294 bbb929c8-8fbe-4397-9dbb-9b2b20218538