Bug 1468128: Use an extra bit for the content process portion of MSAA unique ids. r=aklotz

This allows us to support 255 content processes instead of 127.
It also means we have 1 less bit for the Accessible portion of the id, which means we can support less Accessibles per process.
Hopefully, this shouldn't be a problem, especially with Fission generally meaning less documents per process.

Differential Revision: https://phabricator.services.mozilla.com/D104344

accessible/ipc/win/handler/AccessibleHandler.cpp

@@ -47,9 +47,9 @@ const WCHAR kEmulatedWindowClassName[] = L"MozillaContentWindowClass";
 const uint32_t kEmulatedWindowClassNameNChars =
     sizeof(kEmulatedWindowClassName) / sizeof(WCHAR);
 // Mask to get the content process portion of a Windows accessible unique id.
-// This is bits 24 through 30 (LSB 0) of the id. This must be kept in sync
+// This is bits 23 through 30 (LSB 0) of the id. This must be kept in sync
 // with kNumContentProcessIDBits in accessible/windows/msaa/MsaaIdGenerator.cpp.
-const uint32_t kIdContentProcessMask = 0x7F000000;
+const uint32_t kIdContentProcessMask = 0x7F800000;
 
 static mscom::Factory<AccessibleHandler> sHandlerFactory;
 

accessible/windows/msaa/MsaaIdGenerator.cpp

@@ -28,7 +28,7 @@ static const uint32_t kNumFullIDBits = 31UL;
 // maximum permitted number of e10s content processes. If the e10s maximum
 // number of content processes changes, then kNumContentProcessIDBits must also
 // be updated if necessary to accommodate that new value!
-static const uint32_t kNumContentProcessIDBits = 7UL;
+static const uint32_t kNumContentProcessIDBits = 8UL;
 static const uint32_t kNumUniqueIDBits = (31UL - kNumContentProcessIDBits);
 
 static_assert(