Remove inconsistent throw() specifications in openvr

Building Chrome with /permissive- (a fussy variant in the VC++ compiler)
reports warnings in openvr because exception specifications were removed
from jsoncpp.cpp but not from json.h. The error is:

    jsoncpp.cpp(2599): warning C5043: 'Json::Exception::what':
    exception specification does not match previous declaration

Removing the throw() specifications from json.h restores consistency and
is what was originally intended.

Change-Id: I6146b6299e23f11b35bc1aefd9640541bbbbe953
Reviewed-on: https://chromium-review.googlesource.com/794830Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519949}

third_party/openvr/README.chromium

@@ -14,6 +14,7 @@ The OpenVR API supports HTC Vive and other headsets for VR.
 Local Modifications:
 Only contains a subset of the git repo necessary to build Chromium.  jsoncpp.cpp
 has been modified to remove exceptions.
+json.h has been modified to remove exceptions (delete "throw()").
 openvr.h has been modified to remove dllimport/dllexport when building a
 non-component build so this can be built as a static library.
 

third_party/openvr/src/src/json/json.h

@@ -425,8 +425,8 @@ namespace Json {
 class JSON_API Exception : public std::exception {
 public:
   Exception(std::string const& msg);
-  ~Exception() throw();
-  char const* what() const throw();
+  ~Exception();
+  char const* what() const;
 protected:
   std::string msg_;
 };