Migrate const std::string& parameters to absl::string_view.

THIS CHANGE IS BELIEVED SAFE

Templated asynchronous code can change the lifetime of string data as a result
of this change; however, the most common uses of these (lambdas and callbacks)
are excluded from this change.
Further, your TAP tests pass.

go/string-ref-to-string-view-lsc

Tested:
    TAP --sample ran all affected tests and none failed
    http://test/OCL:319219698:BASE:319213711:1593616691507:a583f044
PiperOrigin-RevId: 319286243

Author: Googler

app/rest/tests/zlibwrapper_unittest.cc

@@ -27,6 +27,7 @@
 #include "gmock/gmock.h"
 #include "absl/base/macros.h"
 #include "absl/strings/escaping.h"
+#include "absl/strings/string_view.h"
 #include "util/random/acmrandom.h"
 
 // 1048576 == 2^20 == 1 MB
@@ -105,7 +106,7 @@ REGISTER_MODULE_INITIALIZER(zlibwrapper_unittest, {
       << "  Reason: " << limiter.reason();
 });
 
-bool ReadFileToString(const std::string& filename, std::string* output,
+bool ReadFileToString(absl::string_view filename, std::string* output,
                       int64 max_size) {
   std::ifstream f;
   f.open(filename);
@@ -696,7 +697,7 @@ class ZLibWrapperTest : public ::testing::TestWithParam<std::string> {
     return dict;
   }
 
-  std::string ReadFileToTest(const std::string& filename) {
+  std::string ReadFileToTest(absl::string_view filename) {
     std::string uncompbuf;
     LOG(INFO) << "Testing file: " << filename;
     CHECK(ReadFileToString(filename, &uncompbuf, MAX_BUF_SIZE));

app/tests/base64_openssh_test.cc

@@ -18,6 +18,7 @@
 #include "app/src/log.h"
 #include "gtest/gtest.h"
 #include "gmock/gmock.h"
+#include "absl/strings/string_view.h"
 #include "openssl/base64.h"
 
 namespace firebase {
@@ -31,7 +32,7 @@ size_t OpenSSHEncodedLength(size_t input_size) {
   return length;
 }
 
-bool OpenSSHEncode(const std::string& input, std::string* output) {
+bool OpenSSHEncode(absl::string_view input, std::string* output) {
   size_t base64_length = OpenSSHEncodedLength(input.size());
   output->resize(base64_length);
   if (EVP_EncodeBlock(reinterpret_cast<uint8_t*>(&(*output)[0]),
@@ -52,7 +53,7 @@ size_t OpenSSHDecodedLength(size_t input_size) {
   return length;
 }
 
-bool OpenSSHDecode(const std::string& input, std::string* output) {
+bool OpenSSHDecode(absl::string_view input, std::string* output) {
   size_t decoded_length = OpenSSHDecodedLength(input.size());
   output->resize(decoded_length);
   if (EVP_DecodeBase64(reinterpret_cast<uint8_t*>(&(*output)[0]),