[wasm] Turn on parallel compilation by default.

This CL sets the default value of the flag wasm_num_compilation_tasks to 10.
The actual number of compilation tasks is the minimum of the flag value and
V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads(), which is 8
on my machine.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2054093002
Cr-Commit-Position: refs/heads/master@{#36892}

src/flag-definitions.h

@@ -482,7 +482,7 @@ DEFINE_BOOL(turbo_stress_instruction_scheduling, false,
 
 // Flags for native WebAssembly.
 DEFINE_BOOL(expose_wasm, false, "expose WASM interface to JavaScript")
-DEFINE_INT(wasm_num_compilation_tasks, 0,
+DEFINE_INT(wasm_num_compilation_tasks, 10,
            "number of parallel compilation tasks for wasm")
 DEFINE_BOOL(trace_wasm_encoder, false, "trace encoding of wasm code")
 DEFINE_BOOL(trace_wasm_decoder, false, "trace decoding of wasm code")