[wasm] Disable parallel compilation in predictable mode.

R=bradnelson@chromium.org,ishell@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2570543002
Cr-Commit-Position: refs/heads/master@{#41652}

src/flag-definitions.h

@@ -1181,6 +1181,7 @@ DEFINE_IMPLICATION(print_all_code, trace_codegen)
 DEFINE_BOOL(predictable, false, "enable predictable mode")
 DEFINE_IMPLICATION(predictable, single_threaded)
 DEFINE_NEG_IMPLICATION(predictable, memory_reducer)
+DEFINE_VALUE_IMPLICATION(single_threaded, wasm_num_compilation_tasks, 0)
 
 //
 // Threading related flags.