[Cronet] Fix use after free in Cronet PKP test on iOS.

Bug: 670689
Change-Id: I7d09af8130233525bcbf41443266b40709e8b99a
Reviewed-on: https://chromium-review.googlesource.com/568805Reviewed-by: Andrei Kapishnikov <kapishnikov@chromium.org>
Commit-Queue: Misha Efimov <mef@chromium.org>
Cr-Commit-Position: refs/heads/master@{#490816}

components/cronet/ios/test/cronet_pkp_test.mm

@@ -95,8 +95,8 @@ class PkpTest : public CronetTestBase {
   // certificate.
   static NSData* NonMatchingHash() {
     const int length = 32;
-    const char* hash = std::string(length, '\077').c_str();
-    return [NSData dataWithBytes:hash length:length];
+    std::string hash(length, '\077');
+    return [NSData dataWithBytes:hash.c_str() length:length];
   }
 
   // Returns hash value that matches the hash of the public key certificate used