Add dawn_jsoncpp_dir to build_overrides/dawn.gni

jsoncpp will be added as a DEP of Dawn to dump perf test output

Bug: dawn:208
Change-Id: I0cfc3d9caebbcffb6ff68ecd98f6bd1ad4cbf873
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1863356Reviewed-by: John Budorick <jbudorick@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706534}

build_overrides/dawn.gni

@@ -6,6 +6,7 @@
 dawn_glfw_dir = "//third_party/glfw/src"
 dawn_jinja2_dir = "//third_party/jinja2"
 dawn_googletest_dir = "//third_party/googletest/src"
+dawn_jsoncpp_dir = "//third_party/jsoncpp"
 dawn_shaderc_dir = "//third_party/shaderc/src"
 dawn_spirv_cross_dir = "//third_party/spirv-cross/spirv-cross"
 dawn_spirv_tools_dir = "//third_party/SPIRV-Tools/src"