Enable building pdf_use_skia in the standalone build.

DEPS

 use_relative_paths = True
 
+vars = {
+  'chromium_git': 'https://chromium.googlesource.com',

[Tom Sepez, 2016/03/16 18:51:51]

As long as you are going to introduce vars, would you be kind enough to update
all of the code below to use an expression like at line 17?

[dsinclair, 2016/03/16 19:35:14]

Done.

+
+  'skia_revision': 'ed854fb897fa134f2f64738a75293502fdcf1af2'
+}
+
 deps = {
   "build/gyp":
     "https://chromium.googlesource.com/external/gyp",
 
   "buildtools":
     "https://chromium.googlesource.com/chromium/buildtools.git@c2f259809d5ede3275df5ea0842f0431990c4f98",
 
+  "third_party/skia":

[Tom Sepez, 2016/03/16 18:51:51]

alphabetize, move to line 28

[dsinclair, 2016/03/16 19:35:13]

Apparently I can't spell.

+    Var('chromium_git') + '/skia.git' + '@' +  Var('skia_revision'),
+
   "testing/corpus":
-     "https://pdfium.googlesource.com/pdfium_tests@bafb069353f96d441c6a37786d18cd9f5407937f",
+    "https://pdfium.googlesource.com/pdfium_tests@bafb069353f96d441c6a37786d18cd9f5407937f",
 
   "testing/gmock":
-     "https://chromium.googlesource.com/external/googlemock.git@29763965ab52f24565299976b936d1265cb6a271",
+    "https://chromium.googlesource.com/external/googlemock.git@29763965ab52f24565299976b936d1265cb6a271",
 
   "testing/gtest":
-     "https://chromium.googlesource.com/external/googletest.git@8245545b6dc9c4703e6496d1efd19e975ad2b038",
+    "https://chromium.googlesource.com/external/googletest.git@8245545b6dc9c4703e6496d1efd19e975ad2b038",
 
   "tools/clang":
     "https://chromium.googlesource.com/chromium/src/tools/clang",
 
   "v8":
     "https://chromium.googlesource.com/v8/v8.git@3c3d7e7be80f45eeea0dc74a71d7552e2afc2985",
 
   "v8/base/trace_event/common":
     "https://chromium.googlesource.com/chromium/src/base/trace_event/common.git@d83d44b13d07c2fd0a40101a7deef9b93b841732",
 
@@ skipping 56 matching lines @@
                 '-s', 'pdfium/buildtools/linux64/clang-format.sha1',
     ],
   },
   {
     # Pull clang if needed or requested via GYP_DEFINES.
     'name': 'clang',
     'pattern': '.',
     'action': ['python', 'pdfium/tools/clang/scripts/update.py', '--if-needed'],
   },
 ]