build: Update mac build parsing to accept repackaged builds.

Most macOS builds are of the form major-minor-build, such as 19D76. In
some cases, packaging errors are fixed by repackaging an existing build.
In those cases, the build gets a packaging version, such as 19E258a
(packaging version 2), or 19E234g (packaging version 8).

This CL updates the parsing code to account for the package version.

Bug: 1049832
Test: gn gen with enable_nacl={on,off} on macOS 19D76.
Change-Id: I3aac7ee56acc46a77b7a7a8ea5b19ea386c5d84d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2096912
Auto-Submit: Victor Costan <pwnall@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748984}

build/config/mac/sdk_info.py

@@ -76,13 +76,17 @@ def FillMachineOSBuild(settings):
   settings['machine_os_build'] = machine_os_build
 
   # The reported build number is made up from the kernel major version number,
-  # a minor version represented as a letter, and a build number.
+  # a minor version represented as a letter, a build number, and an optional
+  # packaging version.
   #
   # For example, the macOS 10.15.3 GM build is 19D76.
   # - 19 is the Darwin kernel that ships with 10.15.
   # - D is minor version 4. 10.15.0 builds had minor version 1.
   # - 76 is the build number. 75 other builds were stamped before GM came out.
-  build_match = re.match(r'^(\d+)([A-Z])(\d+)$', machine_os_build)
+  #
+  # The macOS 10.15.4 beta 5 build is 19E258a. The trailing "a" means the same
+  # build output was packaged twice.
+  build_match = re.match(r'^(\d+)([A-Z])(\d+)([a-z]?)$', machine_os_build)
   assert build_match, "Unexpected macOS build format: %r" % machine_os_build
   settings['machine_os_build_major'] = int(build_match.group(1), 10)