Implement time functions for Fuchsia.

Implement GetProcessCpuTimeNanos and GetThreadCpuTimeNanos for Fuchsia.
This is needed for the tests call_perf_tests and
video_pc_full_stack_tests on Fuchsia.

Bug: fuchsia:115601
Change-Id: Idd10db93d4087d10896ae3fde6abbc37176f625e
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/290920
Reviewed-by: Christoffer Jansson <jansson@google.com>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Sarah Pham <smpham@google.com>
Cr-Commit-Position: refs/heads/main@{#39119}

rtc_base/BUILD.gn

diff --git a/rtc_base/BUILD.gn b/rtc_base/BUILD.gn
index eb80442..4a9a546 100644
--- a/rtc_base/BUILD.gn
+++ b/rtc_base/BUILD.gn
@@ -1848,6 +1848,9 @@
     "//third_party/abseil-cpp/absl/strings",
     "//third_party/abseil-cpp/absl/types:optional",
   ]
+  if (is_fuchsia) {
+    deps += [ "//third_party/fuchsia-sdk/sdk/pkg/zx" ]
+  }
 }
 
 rtc_library("task_queue_for_test") {