Add CaptureContext and its test

client/capture_context_mac.h

Index: client/capture_context_mac.h
diff --git a/client/capture_context_mac.h b/client/capture_context_mac.h
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+// Copyright 2014 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef CRASHPAD_CLIENT_CAPTURE_CONTEXT_MAC_H_
+#define CRASHPAD_CLIENT_CAPTURE_CONTEXT_MAC_H_
+
+#include <mach/mach.h>
+
+#include "build/build_config.h"
+
+namespace crashpad {
+
+#if defined(ARCH_CPU_X86_FAMILY)
+typedef x86_thread_state NativeCPUContext;
+#endif
+
+//! \brief Saves the CPU context.
+//!
+//! The CPU context will be captured as accurately and completely as possible,
+//! containing an atomic snapshot at the point of this function’s return. This
+//! function does not modify any registers.
+//!
+//! \param[out] cpu_context The structure to store the context in.
+//!
+//! \note On x86_64, the value for `%%rdi` will be populated with the address of
+//!     this function’s argument, as mandated by the ABI. If the value of
+//!     `%%rdi` prior to calling this function is needed, it must be obtained
+//!     separately prior to calling this function. For example:
+//!     \code
+//!       uint64_t rdi;
+//!       asm("movq %%rdi, %0" : "=m"(rdi));
+//!     \endcode
+void CaptureContext(NativeCPUContext* cpu_context);
+
+}  // namespace crashpad
+
+#endif  // CRASHPAD_CLIENT_CAPTURE_CONTEXT_MAC_H_