Without this change, the v8::Local<> constructor will be picked up by the
compiler as an option for an implicit cast for any pointer type.  This leads
to bad error messages when accidentally passing an erroneous pointer type to
a function wanting a Local<> (complains about a pointer assignment in Local<>'s
constructor as opposed to a bad type for the parameter of the function being
called) and also causes ambiguity errors where none should exist when calling
overloaded functions (for example a function taking either a std::string or a
v8::Local<v8::Script> cannot be called with a const char * because the compiler
sees both types as being constructable with a const char *).

R=jochen@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1647833005

Cr-Commit-Position: refs/heads/master@{#33602}