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Now that v8 declares V8_USE_PERFETTO as an external definition that
appears whenever including the main V8 headers
(https://chromium-review.googlesource.com/c/v8/v8/+/2621083), we can
remove its definition from gin.

Bug: 1006541
Change-Id: I5b48bedfce1588c078fda0c307c0f345dcb222ba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2622096
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#842468}