Add experimental support for compiling with clang and ASan on Linux.

DEPS

Index: DEPS
diff --git a/DEPS b/DEPS
index 455150d09331c99d84debb2272ae0121c1bd09f5..220f0c6b4224ae4b4b42b3d2868c5ea91111282c 100644
--- a/DEPS
+++ b/DEPS
@@ -16,6 +16,9 @@ deps = {
   "testing/gtest":
      "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@2607e2b06b0be40a4c3f762c1a666a389dc28a99",
 
@@ -76,4 +79,10 @@ hooks = [
                 '-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'],
+  },
 ]