[asm.js] Enable dedicated asm.js parser and validator.

Note that this just switches from the AST-based validator to a dedicated
parser for asm.js modules. The validation of asm.js modules in general
still is predicated by the "--validate-asm" flag, and not enabled by
default yet.

R=clemensh@chromium.org,marja@chromium.org
BUG=v8:6127

Change-Id: Ibd920b03e20ec3c70ee51b79c6c5a2043964fe4f
Reviewed-on: https://chromium-review.googlesource.com/496146Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45078}

src/flag-definitions.h

@@ -555,8 +555,7 @@ DEFINE_BOOL(wasm_break_on_decoder_error, false,
             "debug break when wasm decoder encounters an error")
 
 DEFINE_BOOL(validate_asm, false, "validate asm.js modules before compiling")
-DEFINE_BOOL(fast_validate_asm, false,
-            "validate asm.js modules before compiling")
+DEFINE_BOOL(fast_validate_asm, true, "validate asm.js modules before compiling")
 DEFINE_BOOL(suppress_asm_messages, false,
             "don't emit asm.js related messages (for golden file testing)")
 DEFINE_BOOL(trace_asm_time, false, "log asm.js timing info to the console")