Add hiroshige@ as a platform/loader OWNER

Bug: None
Change-Id: I10debc9f1636250545b4bf9508d2df7ecf0e62e1
Reviewed-on: https://chromium-review.googlesource.com/c/956234Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Nate Chapin <japhet@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611758}

third_party/blink/renderer/platform/loader/OWNERS

+hiroshige@chromium.org
 japhet@chromium.org
 mkwst@chromium.org
 toyoshim@chromium.org