[turbofan] Enable --turbo-osr.

R=yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1035643002

Cr-Commit-Position: refs/heads/master@{#27431}

src/flag-definitions.h

@@ -416,7 +416,7 @@ DEFINE_BOOL(turbo_verify_allocation, DEBUG_BOOL,
             "verify register allocation in TurboFan")
 DEFINE_BOOL(turbo_move_optimization, true, "optimize gap moves in TurboFan")
 DEFINE_BOOL(turbo_jt, true, "enable jump threading in TurboFan")
-DEFINE_BOOL(turbo_osr, false, "enable OSR in TurboFan")
+DEFINE_BOOL(turbo_osr, true, "enable OSR in TurboFan")
 DEFINE_BOOL(turbo_exceptions, false, "enable exception handling in TurboFan")
 DEFINE_BOOL(turbo_stress_loop_peeling, false,
             "stress loop peeling optimization")