Add GetTimeTicksNow() to PlatformSupport.

mojo/edk/embedder/platform_support.h

Index: mojo/edk/embedder/platform_support.h
diff --git a/mojo/edk/embedder/platform_support.h b/mojo/edk/embedder/platform_support.h
index 02186161082252032a601d863a074d0f1294bae1..5b4f0ac23381bb1e9b63db81c213992c48908277 100644
--- a/mojo/edk/embedder/platform_support.h
+++ b/mojo/edk/embedder/platform_support.h
@@ -9,6 +9,7 @@
 
 #include "mojo/edk/embedder/scoped_platform_handle.h"
 #include "mojo/edk/system/system_impl_export.h"
+#include "mojo/public/c/system/types.h"
 #include "mojo/public/cpp/system/macros.h"
 
 namespace mojo {
@@ -23,6 +24,23 @@ class MOJO_SYSTEM_IMPL_EXPORT PlatformSupport {
  public:
   virtual ~PlatformSupport() {}
 
+  // Gets a "time-ticks" value:
+  //   - The value should be nondecreasing with respect to time/causality.
+  //   - The value should be in microseconds (i.e., if a caller runs
+  //     continuously, getting the value twice, their difference should be
+  //     approximately the real time elapsed between the samples, in
+  //     microseconds).
+  //   - The value should be nonnegative.
+  //   - The behaviour of the value if execution is suspended (i.e., the
+  //     computer "sleeps") is undefined (i.e., this is not a real-time clock),
+  //     except that it must remain monotonic.
+  //   - As observable, monotonicity should hold across threads.
+  // If multiple |PlatformSupport| implementations/instances are used in a
+  // single system, all implementations must agree (i.e., respect the above as
+  // if there were only a single |PlatformSupport|).
+  virtual MojoTimeTicks GetTimeTicksNow() = 0;
+
+  // Gets cryptographically-secure (pseudo)random bytes.
   virtual void GetCryptoRandomBytes(void* bytes, size_t num_bytes) = 0;
 
   virtual PlatformSharedBuffer* CreateSharedBuffer(size_t num_bytes) = 0;