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

The following are currently unused in Android, Google3, Chromium, and Mozilla:
  - SkEvent
  - SkTime::GetMSecs
  - SK_TIME_FACTOR (also unused in Skia)
  - SkAutoTime

I left uses of SkMSec more-or-less intact for SkEvent, SkAnimator, and SkInterpolator. SkInterpolator is used in Chromium, so I did not want to change the API. The views/ and animator/ code is crufty, so it didn't seem worthwhile to refactor it. Instead, I added SkEvent::GetMSecsSinceStartup, which is likely to be adequate for use in SampleApp.

I also left SkMSec where it is used to measure a duration rather than a timestamp. With the exception of SkMovie, which is used in Android, all of the uses appear to measure the execution time of a piece of code, which I would hope does not exceed 2^31 milliseconds.

Added skiatest::Timer to support a common idiom in tests where we want to measure the wallclock time in integer milliseconds. (Not used in tests/PathOpsSkpClipTest.cpp because it redefines things in Test.h.)

Removed tabs in tests/StrokerTest.cpp.

BUG=skia:4632
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1811613004

Review URL: https://codereview.chromium.org/1811613004
diff --git a/src/pdf/SkPDFMetadata.cpp b/src/pdf/SkPDFMetadata.cpp
index 5e8c124..118cda3 100644
--- a/src/pdf/SkPDFMetadata.cpp
+++ b/src/pdf/SkPDFMetadata.cpp
@@ -55,7 +55,7 @@
     SkMD5 md5;
     const char uuidNamespace[] = "org.skia.pdf\n";
     md5.write(uuidNamespace, strlen(uuidNamespace));
-    SkMSec msec = SkTime::GetMSecs();
+    double msec = SkTime::GetMSecs();
     md5.write(&msec, sizeof(msec));
     SkTime::DateTime dateTime;
     SkTime::GetDateTime(&dateTime);