Make build_shaderc.py blow away output dir and allow build_type arg to contain either Debug or Release

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1884963002

Review URL: https://codereview.chromium.org/1884963002
diff --git a/gyp/shaderc.gyp b/gyp/shaderc.gyp
index 26af80a..7732e22 100644
--- a/gyp/shaderc.gyp
+++ b/gyp/shaderc.gyp
@@ -8,12 +8,8 @@
   'variables': {
     'variables': { # This is the dreaded nested variables dict so that we can
                    # have dependent variables
+      'shaderc_build_configuration': '<(CONFIGURATION_NAME)',
       'conditions': [
-        [ 'CONFIGURATION_NAME == "Release_Developer"', {
-          'shaderc_build_configuration' : 'Release',
-        }, {
-          'shaderc_build_configuration' : '<(CONFIGURATION_NAME)',
-        }],
         [ 'skia_os == "win"', {
           'shaderc_lib_name' : 'shaderc_combined.lib',
         }, {