Also remove the ResultMode from ToBooleanStub and always return true or
false and use the same mechanism in fullcodegen.  This is in preparation
for adding ToBoolean hints to TurboFan.

Drive-by-fix: We can use the power of the ToBooleanIC in TurboFan now
that the ResultMode is gone (and the runtime always returns true or
false from the miss handler).

R=mstarzinger@chromium.org
BUG=v8:4583
LOG=n

Review URL: https://codereview.chromium.org/1491223002

Cr-Commit-Position: refs/heads/master@{#32524}