Move build/ to build_gyp/.

This CL moves the build/ files to build_gyp/ in anticipation of pulling in
Chromiums build/ directory.

The gyp_pdfium files have been duplicated into both places. Once the bots
are updated we'll remove the build/ versions.

BUG=pdfium:106

Review URL: https://codereview.chromium.org/1900913003
diff --git a/DEPS b/DEPS
index 265f452..d846d20 100644
--- a/DEPS
+++ b/DEPS
@@ -18,7 +18,7 @@
 }
 
 deps = {
-  "build/gyp":
+  "build_gyp/gyp":
     Var('chromium_git') + "/external/gyp",
 
   "buildtools":
@@ -68,7 +68,7 @@
     # A change to a .gyp, .gypi, or to GYP itself should run the generator.
     'name': 'gyp',
     'pattern': '.',
-    'action': ['python', 'pdfium/build/gyp_pdfium'],
+    'action': ['python', 'pdfium/build_gyp/gyp_pdfium'],
   },
   # Pull GN binaries. This needs to be before running GYP below.
   {