Move mac_find_sdk out of gyp directory

Gyp directory is used by a separate repository. Files in that directory
may be ignored by some commands. Move it back into pdfium repository.

BUG=pdfium:442

Review URL: https://codereview.chromium.org/1835943005
diff --git a/build/gyp/tools/mac_find_sdk.py b/build/mac_find_sdk.py
similarity index 100%
rename from build/gyp/tools/mac_find_sdk.py
rename to build/mac_find_sdk.py
diff --git a/build/standalone.gypi b/build/standalone.gypi
index fe4995b..925326b 100644
--- a/build/standalone.gypi
+++ b/build/standalone.gypi
@@ -454,7 +454,7 @@
         ],  # target_conditions
         'variables': {
           'mac_sdk_min': '10.10',
-          'mac_sdk%': '<!(python <(DEPTH)/build/gyp/tools/mac_find_sdk.py <(mac_sdk_min))',
+          'mac_sdk%': '<!(python <(DEPTH)/build/mac_find_sdk.py <(mac_sdk_min))',
         },
         'xcode_settings': {
           'SDKROOT': 'macosx<(mac_sdk)',  # -isysroot