Use Dawn's BUILD.gn instead of defining another one for Skia.

Bug: chromium:1064305

Change-Id: I10ca629ff88a5492bb9f89075f6d92094340ff55
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282076
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Zhenyao Mo <zmo@google.com>

BUILD.gn

@@ -578,11 +578,11 @@ optional("gpu") {
   if (skia_use_dawn) {
     public_defines += [ "SK_DAWN" ]
     sources += skia_dawn_sources
-    public_deps += [ "//third_party/dawn:dawn_headers" ]
+    public_deps += [ "//third_party/externals/dawn/src/dawn:dawn_headers" ]
     deps += [
-      "//third_party/dawn:dawncpp",
-      "//third_party/dawn:libdawn_native",
-      "//third_party/dawn:libdawn_proc",
+      "//third_party/externals/dawn/src/dawn:dawn_proc",
+      "//third_party/externals/dawn/src/dawn:dawncpp",
+      "//third_party/externals/dawn/src/dawn_native",
     ]
     if (dawn_enable_d3d12) {
       libs += [
@@ -1220,7 +1220,7 @@ if (skia_enable_tools) {
     ]
 
     if (skia_use_dawn) {
-      deps += [ "//third_party/dawn:dawn_headers" ]
+      deps += [ "//third_party/externals/dawn/src/dawn:dawn_headers" ]
     }
   }
 
@@ -1385,7 +1385,7 @@ if (skia_enable_tools) {
       sources += [ "tools/gpu/d3d/D3DTestUtils.cpp" ]
     }
     if (skia_use_dawn) {
-      public_deps += [ "//third_party/dawn:dawn_headers" ]
+      public_deps += [ "//third_party/externals/dawn/src/dawn:dawn_headers" ]
       sources += [ "tools/gpu/dawn/DawnTestContext.cpp" ]
       cflags_cc = [ "-Wno-microsoft-cast" ]
     }

build_overrides/dawn.gni

@@ -10,11 +10,10 @@
 # many of these as possible. This will require updating Skia's
 # SPIRV-Tools and Shaderc at a minimum.
 
-# skia/third_party/externals/jinja2 relative to skia/third_party/dawn/BUILD.gn
-dawn_jinja2_dir = "../externals/jinja2"
+# Skia doesn't have Chromium's //build directory. Tell Dawn to do without.
+dawn_has_build = false
 
-# dawn_glfw_dir = "//third_party/glfw"
-# dawn_googletest_dir = "//third_party/googletest"
+dawn_jinja2_dir = "//third_party/externals/jinja2"
 dawn_shaderc_dir = "//third_party/shaderc"
 dawn_spirv_tools_dir = "//third_party/spirv-tools"
 dawn_spirv_cross_dir = "//third_party/spirv-cross"