use absolute path to clang plugins

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.
 
-THIS_ABS_DIR=$(cd $(dirname $0) && echo $PWD)
-CLANG_LIB_PATH=$THIS_ABS_DIR/../../../third_party/llvm-build/Release+Asserts/lib
+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
 LIBNAME=\
-$(grep 'set(LIBRARYNAME' "$THIS_ABS_DIR"/../blink_gc_plugin/CMakeLists.txt \
+$(grep 'set(LIBRARYNAME' "$SRC_DIR"/tools/clang/blink_gc_plugin/CMakeLists.txt \
     | cut -d ' ' -f 2 | tr -d ')')
 
 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"
   fi
 done
 
 echo -Xclang -load -Xclang $CLANG_LIB_PATH/lib$LIBNAME.$LIBSUFFIX \
   -Xclang -add-plugin -Xclang blink-gc-plugin $FLAGS