Require Windows 10 for building Chromium

Windows 7 no longer works for building Chromium and nobody has
volunteered to add support and Windows 7 is an unsupported OS so this
change formalizes that Windows 10 is now required in order to build
Chromium. This updates the documentation and modifies one of the build
scripts so that a readable error message will be displayed when building
on Windows 7 is attempted.

Chromium and its derivatives still run on Windows 7. This only affects
developers.

Bug: 1150302
Change-Id: I1874c691a1405482232323bef025aba4b3ac74b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2570075Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833327}

build/write_buildflag_header.py

@@ -91,5 +91,13 @@ def WriteHeader(options):
     output_file.write('\n#endif  // %s\n' % options.header_guard)
 
 
+if os.name == 'nt':
+  major, minor, build, platform, service_pack = sys.getwindowsversion()
+  # Windows 10 will be 6.2 on Python 2 and 10.0 on Python 3. This check
+  # handles both.
+  if major < 6 or (major == 6 and minor < 2):
+    raise Exception(
+        'Unsupported OS. Building Chromium requires Windows 10. %s detected.' %
+        str(sys.getwindowsversion()))
 options = GetOptions()
 WriteHeader(options)

docs/windows_build_instructions.md

@@ -17,7 +17,7 @@ Are you a Google employee? See
 * At least 100GB of free disk space on an NTFS-formatted hard drive. FAT32
   will not work, as some of the Git packfiles are larger than 4GB.
 * An appropriate version of Visual Studio, as described below.
-* Windows 7 or newer.
+* Windows 10 or newer.
 
 ## Setting up Windows