Add a template-specialized constructor for der::Input for constants

Make it easier to create der::Input's from constants by the magical
power of Templates.

Old and busted: der::Input foo(kFoo, arraysize(kFoo));
New hotness: der::Input foo(kFoo);

BUG=none
R=mattm@chromium.org

Review URL: https://codereview.chromium.org/1146283004

Cr-Commit-Position: refs/heads/master@{#330882}

net/der/input.h

@@ -41,6 +41,11 @@ class NET_EXPORT_PRIVATE Input {
   // Creates an empty Input, one from which no data can be read.
   Input();
 
+  // Creates an Input from a constant array |data|.
+  template <size_t N>
+  explicit Input(const uint8_t(&data)[N])
+      : data_(data), len_(N) {}
+
   // Creates an Input from the given |data| and |len|.
   Input(const uint8_t* data, size_t len);
 

net/der/input_unittest.cc

@@ -27,6 +27,14 @@ TEST(InputTest, Equals) {
   EXPECT_FALSE(test_truncated.Equals(test));
 }
 
+TEST(InputTest, StaticArray) {
+  Input input(kInput);
+  EXPECT_EQ(arraysize(kInput), input.Length());
+
+  Input input2(kInput, arraysize(kInput));
+  EXPECT_TRUE(input.Equals(input2));
+}
+
 TEST(ByteReaderTest, NoReadPastEnd) {
   ByteReader reader(Input(nullptr, 0));
   uint8_t data;