Add dummy DEPS file for jsoncpp.

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

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

Bug: 1017514
Change-Id: I3832b7fadc0fa9ddcc5dad700690b09e374a4f67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1877455Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709541}

third_party/.gitignore

@@ -119,7 +119,6 @@
 /jacoco/lib/
 /javalang/src/
 /jdk/current
-/jsoncpp/source
 /jsr-305/src
 /junit/src
 /khronos_glcts

third_party/jsoncpp/.gitignore

+/source

third_party/jsoncpp/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 jsoncpp_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.
+  'jsoncpp_revision': 'master',
+}
+
+deps = {
+  'source': '{chromium_git}/open-source-parsers/jsoncpp.git@{jsoncpp_revision}'
+}