[ptr-compr] Switch Smis to 31-bit on 64-bit platforms.

This is prerequisite for V8 heap pointer compression.

Bug: v8:7703
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: I2cdf02bd4cd535beb78a5db5b7cbdf67433a6d16
Reviewed-on: https://chromium-review.googlesource.com/1181136
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55269}

include/v8.h

@@ -245,7 +245,7 @@ static_assert(
     "Pointer compression can be enabled only for 64-bit architectures");
 typedef SmiTagging<4> PlatformSmiTagging;
 #else
-typedef SmiTagging<kApiPointerSize> PlatformSmiTagging;
+typedef SmiTagging<4> PlatformSmiTagging;
 #endif
 
 const int kSmiShiftSize = PlatformSmiTagging::kSmiShiftSize;