Provide an explicit constructor for C array initialization of CrashKeyString.

This mirrors
https://chromium-review.googlesource.com/c/crashpad/crashpad/+/807645 in
Crashpad.

Bug: 598854
Change-Id: I1b98dab78df7a30d913be3a476c8b373063e6065
Reviewed-on: https://chromium-review.googlesource.com/820160Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523302}

components/crash/core/common/crash_key.h

@@ -125,9 +125,14 @@ class CrashKeyString : public internal::CrashKeyStringImpl {
   constexpr static size_t chunk_count =
       (MaxLength / internal::kCrashKeyStorageValueSize) + 1;
 
+  // A constructor tag that can be used to initialize a C array of crash keys.
+  enum class Tag { kArray };
+
   constexpr explicit CrashKeyString(const char name[])
       : internal::CrashKeyStringImpl(name, indexes_.data, chunk_count) {}
 
+  constexpr CrashKeyString(const char name[], Tag tag) : CrashKeyString(name) {}
+
  private:
   // Indexes into the TransitionalCrashKeyStorage for when a value is set.
   // See the comment in CrashKeyStringImpl for details.

components/crash/core/common/crash_key_unittest.cc

@@ -98,5 +98,23 @@ TEST_F(CrashKeyStringTest, BaseSupport) {
   base::debug::SetCrashKeyString(crash_key, std::string(64, 'a'));
 }
 
+TEST_F(CrashKeyStringTest, CArrayInitializer) {
+  static CrashKeyString<8> keys[] = {
+      {"test-1", CrashKeyString<8>::Tag::kArray},
+      {"test-2", CrashKeyString<8>::Tag::kArray},
+      {"test-3", CrashKeyString<8>::Tag::kArray},
+  };
+
+  EXPECT_FALSE(keys[0].is_set());
+  EXPECT_FALSE(keys[1].is_set());
+  EXPECT_FALSE(keys[2].is_set());
+
+  keys[1].Set("test");
+
+  EXPECT_FALSE(keys[0].is_set());
+  EXPECT_TRUE(keys[1].is_set());
+  EXPECT_FALSE(keys[2].is_set());
+}
+
 }  // namespace
 }  // namespace crash_reporter