Add dummy DEPS file for patched YASM.

This will allow clients that use the subtree mirror to specify a
revision for patched YASM without needing nested DEPS.

Also cleans up the gitignore for the subtree mirror so gclient
revert will work properly.

Bug: 1017514
Change-Id: I7a3a51d1ab79437dd315a246b833211d0ef08df3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877810Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709539}

third_party/.gitignore

@@ -248,5 +248,4 @@
 /xulrunner-sdk
 /yasm/binaries
 /yasm/generate_files.xml
-/yasm/source/patched-yasm
 /yasm/yasm.xml

third_party/yasm/.gitignore

+/source/patched-yasm

third_party/yasm/DEPS

+# This file is a dummy used so that non-Chromium clients can specify
+# recursive DEPS. Otherwise the clients would need to nest DEPS inside
+# each other. Nested DEPS are not supported by gclient.
+#
+# Clients *must* specify patched_yasm_revision when using this DEPS file.
+
+use_relative_paths = True
+
+vars = {
+  'chromium_git': 'https://chromium.googlesource.com',
+
+  # We must specify a dummy variable here for recursedeps to work.
+  'patched_yasm_revision': 'master',
+}
+
+deps = {
+  'source/patched-yasm': '{chromium_git}/chromium/deps/yasm/patched-yasm.git@{jsoncpp_revision}'
+}