Make macOS OS version numbers consistent

The "IsOS/IsAtLeastOS/IsAtMostOS" functions are driven from the Darwin
version number, which is always accurate. Adjust the
SysInfo::OperatingSystemVersionNumbers() function to return accurate
version numbers (or the best we can deduce) so that all version
numbers returned by helper functions are accurate and consistent.

Bug: 1101439
Bug: 1108832
Change-Id: I2ca92478d76bf572cc55875cba443f4978482d10
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2315490Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791491}

base/system/sys_info_mac.mm

@@ -61,6 +61,19 @@ void SysInfo::OperatingSystemVersionNumbers(int32_t* major_version,
   *major_version = version.majorVersion;
   *minor_version = version.minorVersion;
   *bugfix_version = version.patchVersion;
+
+  // TODO(https://crbug.com/1108832): If an app is built against a pre-macOS
+  // 11.0 SDK, macOS will lie as to what version it is, saying that it is macOS
+  // "10.16" rather than "11.0". The problem is that the "IsOS/IsAtLeastOS/
+  // IsAtMostOS" functions are driven from the Darwin version number, which
+  // isn't lied about, and therefore the values returned by this function and
+  // those functions are inconsistent. Therefore, unlie about these values.
+
+  if (*major_version == 10 && *minor_version >= 16) {
+    *major_version = *minor_version - 5;
+    *minor_version = *bugfix_version;
+    *bugfix_version = 0;
+  }
 }
 
 // static