Support custom path of blink_gc_plugin for distributed builds.

tools/clang/scripts/blink_gc_plugin_flags.sh

 #!/usr/bin/env bash
 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script returns the flags that should be passed to clang.
 
 SRC_DIR=$(cd $(dirname $0)/../../.. && echo $PWD)
 CLANG_LIB_PATH=$SRC_DIR/third_party/llvm-build/Release+Asserts/lib
 
 if uname -s | grep -q Darwin; then
   LIBSUFFIX=dylib
 else
   LIBSUFFIX=so
 fi
 
 FLAGS=""
 PREFIX="-Xclang -plugin-arg-blink-gc-plugin -Xclang"
 for arg in "$@"; do
   if [[ "$arg" = "enable-oilpan=1" ]]; then
     FLAGS="$FLAGS $PREFIX enable-oilpan"
   elif [[ "$arg" = "dump-graph=1" ]]; then
     FLAGS="$FLAGS $PREFIX dump-graph"
   elif [[ "$arg" = "warn-raw-ptr=1" ]]; then
     FLAGS="$FLAGS $PREFIX warn-raw-ptr"
   elif [[ "$arg" = "warn-unneeded-finalizer=1" ]]; then
     FLAGS="$FLAGS $PREFIX warn-unneeded-finalizer"
+  elif [[ "$arg" = custom_clang_lib_path=* ]]; then
+    CLANG_LIB_PATH="$(cd "${arg#*=}" && echo $PWD)"
   fi
 done
 
-echo -Xclang -load -Xclang $CLANG_LIB_PATH/libBlinkGCPlugin.$LIBSUFFIX \
+echo -Xclang -load -Xclang \"$CLANG_LIB_PATH/libBlinkGCPlugin.$LIBSUFFIX\" \
   -Xclang -add-plugin -Xclang blink-gc-plugin $FLAGS