Simplify tools/valgrind/locate_valgrind.sh: only allow whitelisted platforms

TEST=./tools/valgrind/chrome_tests.sh -t cmdline -- echo "hello"

Review URL: https://codereview.chromium.org/966363003

Cr-Commit-Position: refs/heads/master@{#318904}

tools/valgrind/locate_valgrind.sh

@@ -36,14 +36,13 @@ then
     PLATFORM="mac_10.7"
     ;;
   *)
-    echo "Unknown platform:" >&2
-    uname -a >&2
-    echo "We'll try to search for valgrind binaries installed in /usr/local" >&2
-    PLATFORM=
+    (echo "Sorry, your platform is not supported:" &&
+     uname -a
+     echo
+     echo "If you're on Mac OS X, please see http://crbug.com/441425") >&2
+    exit 42
   esac
 
-  if [ "$PLATFORM" != "" ]
-  then
   # The binaries should be in third_party/valgrind
   # (checked out from deps/third_party/valgrind/binaries).
   CHROME_VALGRIND="$THISDIR/../../third_party/valgrind/$PLATFORM"
@@ -54,14 +53,7 @@ then
     [ "$PLATFORM" != "mac_10.7" ]
   then
     # Get rid of all "../" dirs
-      CHROME_VALGRIND=`readlink -f $CHROME_VALGRIND`
-    fi
-
-    if ! test -x $CHROME_VALGRIND/bin/valgrind
-    then
-      # We couldn't find the binaries in third_party/valgrind
-      CHROME_VALGRIND=""
-    fi
+    CHROME_VALGRIND=$(readlink -f $CHROME_VALGRIND)
   fi
 fi