Remove an unused field from WebExternalTextureMailbox.h.

gpuMemoryBufferId is no longer necessary.

BUG=

Review-Url: https://codereview.chromium.org/2132913004
Cr-Commit-Position: refs/heads/master@{#404560}

third_party/WebKit/public/platform/WebExternalTextureMailbox.h

@@ -41,7 +41,6 @@ struct WebExternalTextureMailbox {
     bool validSyncToken = false;
     bool allowOverlay = false;
     bool nearestNeighbor = false;
-    int gpuMemoryBufferId = -1;
     unsigned textureTarget = 0;
     WebSize textureSize;