Re-enable 3 clang warnings

Bug: skia:7462
Change-Id: Iba0af2f0ff8ef646f93a0e2ced083d99020a38c9
Reviewed-on: https://skia-review.googlesource.com/90842
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
diff --git a/gn/BUILD.gn b/gn/BUILD.gn
index c94d237..3bc4150 100644
--- a/gn/BUILD.gn
+++ b/gn/BUILD.gn
@@ -313,18 +313,15 @@
       "-Wno-documentation-unknown-command",
       "-Wno-double-promotion",
       "-Wno-exit-time-destructors",  # TODO: OK outside libskia
-      "-Wno-float-conversion",
       "-Wno-float-equal",
       "-Wno-format-nonliteral",
       "-Wno-global-constructors",  # TODO: OK outside libskia
-      "-Wno-gnu-zero-variadic-macro-arguments",
       "-Wno-missing-prototypes",
       "-Wno-missing-variable-declarations",
       "-Wno-pedantic",
       "-Wno-reserved-id-macro",
       "-Wno-shadow",
       "-Wno-shift-sign-overflow",
-      "-Wno-sign-conversion",
       "-Wno-signed-enum-bitfield",
       "-Wno-switch-enum",
       "-Wno-undef",