Some IdentifierNames are only included in the set of FutureReservedWords
for strict mode code. Outside of strict mode, these IdentifierNames may
be used as Identifiers. Notably, this includes their use as
BindingIdentifiers in LexicalBindings.

From ES2015 12.1.1 Static Semantics: Early Errors (Identifiers):

> It is a Syntax Error if this phrase is contained in strict mode code
> and the StringValue of IdentifierName is: "implements", "interface",
> "let", "package", "private", "protected", "public", "static", or
> "yield".

http://www.ecma-international.org/ecma-262/6.0/#sec-identifiers-static-semantics-early-errors

Due to a error in its heuristic for disambiguating the `let` token, V8
does not currently allow any of the strict-mode-only FutureReservedWords
to be used as a BindingIdentifier outside of strict mode.

Update V8's heuristic for disambiguating the `let` keyword to account
for strict mode, enabling these IdentifierNames to be used

BUG=v8:4918
LOG=N
R=adamk@chromium.org

Review-Url: https://codereview.chromium.org/1891453005
Cr-Commit-Position: refs/heads/master@{#36296}