Ignore PNG chunk CRCs in Rust PNG fuzzing builds

`rust/png/FFI.rs` already skips chunk CRC checks under
`cfg!(FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION)`, but nothing in Skia's
GN -> Bazel build set that cfg. As a result, fuzzers built with
`skia_build_fuzzers=true` rejected almost every mutated input with a CRC
`FormatError` before reaching the decoding logic.

Add a `//rust/png:fuzzing_build_mode` Bazel flag (mirroring
`//rust/png:for_android`) that passes the cfg to rustc, and set it from
GN when `skia_build_fuzzers` is true.

Bug: b/562799124
Change-Id: Iaa5e23396bd14eaeb59782af4e8297264a14b625
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1379816
Reviewed-by: Łukasz Anforowicz <lukasza@google.com>
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Elliot Sisteron <elliotsisteron@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 0663bb5..d5d717d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -1476,6 +1476,11 @@
     } else {
       bazel_args += [ "--//rust/png:for_android=false" ]
     }
+    if (skia_build_fuzzers) {
+      bazel_args += [ "--//rust/png:fuzzing_build_mode=true" ]
+    } else {
+      bazel_args += [ "--//rust/png:fuzzing_build_mode=false" ]
+    }
   }
 
   if (skia_use_rust_bmp_decode) {

rust/png/BUILD.bazel

diff --git a/rust/png/BUILD.bazel b/rust/png/BUILD.bazel
index 4c53452..e68d803 100644
--- a/rust/png/BUILD.bazel
+++ b/rust/png/BUILD.bazel
@@ -19,6 +19,13 @@
     default = True,
 )
 
+# Set for fuzzing builds (e.g. `skia_build_fuzzers=true` in GN), so that `FFI.rs`
+# skips checks such as chunk CRCs that would otherwise reject most mutated inputs.
+bool_flag(
+    name = "fuzzing_build_mode",
+    default = False,
+)
+
 skia_filegroup(
     name = "rs_srcs",
     srcs = ["FFI.rs"],
@@ -64,7 +71,10 @@
         "//conditions:default": [],
     }),
     crate_root = "FFI.rs",
-    rustc_flags = ["-Dwarnings"],
+    rustc_flags = ["-Dwarnings"] + select({
+        ":fuzzing_build_mode_true": ["--cfg=FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION"],
+        "//conditions:default": [],
+    }),
     visibility = [
         "//src/codec:__pkg__",
         "//src/encode:__pkg__",