Add FileReaderInterface::ReadExactly() and FileSeekerInterface::SeekSet()

util/file/file_reader.cc

Index: util/file/file_reader.cc
diff --git a/util/file/file_reader.cc b/util/file/file_reader.cc
index b6fd0173e262b294f4acb67f1cc85781bdbfe7b9..c983f4f6e9ba0b22940f125c7a74c1a898ad2d15 100644
--- a/util/file/file_reader.cc
+++ b/util/file/file_reader.cc
@@ -19,6 +19,20 @@
 
 namespace crashpad {
 
+bool FileReaderInterface::ReadExactly(void* data, size_t size) {
+  ssize_t expect = base::checked_cast<ssize_t>(size);
+  ssize_t rv = Read(data, size);
+  if (rv < 0) {
+    // Read() will have logged its own error.
+    return false;
+  } else if (rv != expect) {
+    LOG(ERROR) << "ReadExactly(): expected " << expect << ", observed " << rv;
+    return false;
+  }
+
+  return true;
+}
+
 WeakFileHandleFileReader::WeakFileHandleFileReader(FileHandle file_handle)
     : file_handle_(file_handle) {
 }