Add DEPS.chromium for recursive DEPS tracking.

DEPS.chromium allows the Chromium build system's DEPS to recurse into V8's own
dependencies. Initially, this is populated with some tests files for the ARM64
simulator.

BUG=chromium:718439

Review-Url: https://codereview.chromium.org/2880293002
Cr-Commit-Position: refs/heads/master@{#45310}

DEPS.chromium

+# This file is used by the Chromium dependency system to pull in V8 dependencies
+# not used by Chromium directly.
+#
+# If you make a change to the dependencies below, the corresponding DEPS file
+# for v8 must also be changed.
+
+vars = {
+  "chromium_url": "https://chromium.googlesource.com",
+}
+
+deps = {
+  "src/v8/test/cctest/traces-arm64":
+    Var("chromium_url") + "/external/git.linaro.org/arm/vixl-simulator-traces.git" + "@" + "6168e7e1eec52c9cb0a62f87f94df0582dc48aa8",
+}