gn: leave PKG_CONFIG_PATH untouched when not using a sysroot

build/config/linux/pkg-config.py

Index: build/config/linux/pkg-config.py
diff --git a/build/config/linux/pkg-config.py b/build/config/linux/pkg-config.py
index b0381583c2cad6b0a411da1ed52bc2a24ddf7d73..deaeed3a977495aa3241ad4a402715bde65cac7f 100644
--- a/build/config/linux/pkg-config.py
+++ b/build/config/linux/pkg-config.py
@@ -44,8 +44,7 @@ def SetConfigPath(options):
   options on the given command line."""
 
   sysroot = options.sysroot
-  if not sysroot:
-    sysroot = ""
+  assert sysroot
 
   # Compute the library path name based on the architecture.
   arch = options.arch
@@ -122,8 +121,8 @@ if options.strip_out != None:
   for regexp in options.strip_out:
     strip_out.append(re.compile(regexp))
 
-SetConfigPath(options)
 if options.sysroot:
+  SetConfigPath(options)
   prefix = GetPkgConfigPrefixToStrip(args)
 else:
   prefix = ''