SuperSize: Fix merge conflict in wasmbuild.patch

And tweak README.md build instructions

Bug: None
Change-Id: Ic092e33392d30b27dd3666593c1e0d601b5617c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032804Reviewed-by: Samuel Huang <huangs@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737801}

tools/binary_size/BUILD.gn

@@ -26,3 +26,10 @@ if (is_linux) {
     deps = [ "//tools/binary_size/libsupersize/caspian:cli($host_toolchain)" ]
   }
 }
+
+# is_wasm is defined in BUILDCONFIG.gn only when wasmbuild.patch is applied.
+if (defined(is_wasm)) {
+  group("caspian_web") {
+    deps = [ "//tools/binary_size/libsupersize/caspian:caspian_web(//build/toolchain/wasm:wasm)" ]
+  }
+}

tools/binary_size/libsupersize/caspian/README.md

@@ -12,8 +12,7 @@ https://emscripten.org/docs/getting_started/downloads.html
 ```sh
 git apply tools/binary_size/libsupersize/caspian/wasmbuild.patch
 gn gen out/caspian --args='is_official_build=true treat_warnings_as_errors=false fatal_linker_warnings=false'
-autoninja -C out/caspian tools/binary_size:caspian_web
-cp out/caspian/wasm/caspian_web.* tools/binary_size/libsupersize/static/
+( cd out/caspian; autoninja tools/binary_size:caspian_web && cp wasm/caspian_web.* ../../tools/binary_size/libsupersize/static/ )
 ```
 
 Then run locally via:

tools/binary_size/libsupersize/caspian/wasmbuild.patch

@@ -88,17 +88,3 @@ index 000000000000..15bc7a4ea258
 +    "{{output_dir}}/{{target_output_name}}.wasm.map",
 +  ]
 +}
-diff --git a/tools/binary_size/BUILD.gn b/tools/binary_size/BUILD.gn
-index a9629c27bc42..aa07d6376ac7 100644
---- a/tools/binary_size/BUILD.gn
-+++ b/tools/binary_size/BUILD.gn
-@@ -30,3 +30,9 @@ if (is_linux) {
-     ]
-   }
- }
-+
-+group("caspian_web") {
-+  deps = [
-+    "//tools/binary_size/libsupersize/caspian:caspian_web(//build/toolchain/wasm:wasm)",
-+  ]
-+}