clean up use of register storage class

This fixes this warning-as-error, so we don't have to stifle it any more:
    error: 'register' storage class specifier is deprecated and incompatible with C++1z [-Werror,-Wdeprecated-register]

Tested by building for mipsel via GN.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2289293002
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Review-Url: https://codereview.chromium.org/2289293002
diff --git a/BUILD.gn b/BUILD.gn
index 7f16f7b..446345b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -232,7 +232,7 @@
 opts("dsp") {
   enabled = current_cpu == "mipsel"
   sources = opts_gypi.mips_dsp_sources
-  cflags = [ "-Wno-deprecated-register" ]  # FIXME
+  cflags = []
 }
 
 # Any feature of Skia that requires third-party code should be optional and use this template.