Reland "[wasm] Enable --wasm-shared-engine by default."

This is a reland of c5c447d4

Original change's description:
> [wasm] Enable --wasm-shared-engine by default.
> 
> R=titzer@chromium.org
> BUG=v8:7424
> 
> Change-Id: I3bb3be92b6c1bf855904cf0bf334ad98dce74dd4
> Reviewed-on: https://chromium-review.googlesource.com/1156310
> Reviewed-by: Ben Titzer <titzer@chromium.org>
> Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54855}

Bug: v8:7424
Change-Id: I58153f05bd1eb764f5e58968de1daaaed79215dc
Reviewed-on: https://chromium-review.googlesource.com/1160361Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54871}

src/flag-definitions.h

@@ -614,7 +614,7 @@ DEFINE_BOOL(wasm_no_bounds_checks, false,
 DEFINE_BOOL(wasm_no_stack_checks, false,
             "disable stack checks (performance testing only)")
 
-DEFINE_BOOL(wasm_shared_engine, false,
+DEFINE_BOOL(wasm_shared_engine, true,
             "shares one wasm engine between all isolates within a process")
 DEFINE_IMPLICATION(future, wasm_shared_engine)
 DEFINE_BOOL(wasm_shared_code, false,