optimize-png-files.sh: relax cwd restriction

There's nothing in this script that really requires being run in the
top level of the src.git repo other than the git commit logic.  That
means we can run this script completely outside of src.git if given
a set of paths to operate on.  Move the check into the git scenario.

Bug: None
Test: ran the script outside of src.git on some png files
Change-Id: I4d1343f3757a05e8675ac80c618c8b4ec23e1695
Reviewed-on: https://chromium-review.googlesource.com/1097615
Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566690}

tools/resources/optimize-png-files.sh

@@ -419,11 +419,6 @@ Options:
   exit 1
 }
 
-if [ ! -e ../.gclient ]; then
-  echo "$0 must be run in src directory"
-  exit 1
-fi
-
 if [ "$(expr substr $(uname -s) 1 6)" == "CYGWIN" ]; then
   using_cygwin=true
 else
@@ -512,6 +507,12 @@ set ${DIRS:=$ALL_DIRS}
 info "Optimize level=$OPTIMIZE_LEVEL"
 
 if [ -n "$COMMIT" ] ; then
+  # To keep git logic below sane, require it be run from the top dir.
+  if [ ! -e ../.gclient ]; then
+    echo "$0 must be run in src directory"
+    exit 1
+  fi
+
  ALL_FILES=$(git diff --name-only $COMMIT HEAD $DIRS | grep "png$")
  ALL_FILES_LIST=( $ALL_FILES )
  echo "Processing ${#ALL_FILES_LIST[*]} files"