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
2 files changed