Change SkTime::GetMSecs to double; ensure values stored in SkMSec do not overflow.

tests/Test.cpp

Index: tests/Test.cpp
diff --git a/tests/Test.cpp b/tests/Test.cpp
index f6f2bdab3d751053f82d14811cb0840185af73dc..dcb08e6badc6bf513740657f01e733c46794f559 100644
--- a/tests/Test.cpp
+++ b/tests/Test.cpp
@@ -36,3 +36,17 @@ SkString skiatest::GetTmpDir() {
     const char* tmpDir = FLAGS_tmpDir.isEmpty() ? nullptr : FLAGS_tmpDir[0];
     return SkString(tmpDir);
 }
+
+skiatest::Timer::Timer() : fStartNanos(SkTime::GetNSecs()) {}
+
+double skiatest::Timer::elapsedNs() const {
+    return SkTime::GetNSecs() - fStartNanos;
+}
+
+double skiatest::Timer::elapsedMs() const { return this->elapsedNs() * 1e-6; }
+
+SkMSec skiatest::Timer::elapsedMsInt() const {
+    const double elapsedMs = this->elapsedMs();
+    SkASSERT(SK_MSecMax >= elapsedMs);
+    return static_cast<SkMSec>(elapsedMs);
+}