Use lkgr revision of Clang for PDFium

Recent change of Clang (https://codereview.chromium.org/1879753002)
invokes GetVSVersion() which imports vs_toolchain which PDFium has
not used yet. This breaks our bots. Now we pinned Clang to last good
version.

We can either change back to use Tot Clang after we have toolchain
utility, or we can roll Clang periodically.

Review URL: https://codereview.chromium.org/1900823002
diff --git a/DEPS b/DEPS
index 595da73..465a190 100644
--- a/DEPS
+++ b/DEPS
@@ -5,6 +5,7 @@
   'pdfium_git': 'https://pdfium.googlesource.com',
 
   'buildtools_revision': 'c2f259809d5ede3275df5ea0842f0431990c4f98',
+  'clang_revision': '87058e09f9c547eb5d00cb8ca666c6aec203a117',
   'cygwin_revision': 'c89e446b273697fadf3a10ff1007a97c0b7de6df',
   'gmock_revision': '29763965ab52f24565299976b936d1265cb6a271',
   'gtest_revision': '8245545b6dc9c4703e6496d1efd19e975ad2b038',
@@ -36,7 +37,7 @@
     Var('chromium_git') + '/skia.git' + '@' +  Var('skia_revision'),
 
   "tools/clang":
-    Var('chromium_git') + "/chromium/src/tools/clang",
+    Var('chromium_git') + "/chromium/src/tools/clang@" +  Var('clang_revision'),
 
   "v8":
     Var('chromium_git') + "/v8/v8.git@" + Var('v8_revision'),