Fixed bugs in 20190901094958

Bug 1491456 R[Core:Widget: Cocoa]When using the CoreAnimation present path, consider tiling the window into multiple CALayers for a weak form of partial present
Bug 1547676 R[Firefox:Address Bar]Add in-tree ‘Guide to implementing QB WebExtensions’ documentation
Bug 1560378 R[Core:Document Navigation]Intermittent docshell/test/navigation/test_bug1364364.html | check history.length after bfcache dropped - got 6, expected 2
Bug 1569728 N[Firefox Build System:General]compile clang with GCC 7
Bug 1572470 R[Firefox:Search]Add wikipedia to engines.json
Bug 1572698 V[Toolkit:Password Manager]The Website Address link is wrongly displayed in the left part on RTL builds
Bug 1574586 R[Core:Graphics: Layers]Avoid refreshing the vibrancy on every composite by compositing into multiple framebuffers rather than sharing one framebuffer for the entire window between layers
Bug 1575681 R[Firefox:Enterprise Policies]Provide a policy to disable the password manager altogether
Bug 1575906 R[Toolkit:Crash Reporting]Crash annotations don't work in GeckoMediaPlugin processes on Windows
Bug 1576507 R[Toolkit:General]provide `FirstStartup` service for post-install tasks, starting with Normandy
Bug 1576748 R[Firefox Build System:General]make symbolstore.py complain loudly if expected output isn't found
Bug 1577236 R[Firefox Build System:Source Code Analysis]clang-10: Fix -Wimplicit-int-float-conversion warnings
Bug 1577700 R[Core:MFBT]Add a function for obtaining a Span of char from a Span of uint8_t
Bug 1577790 R[DevTools:Accessibility Tools]Dark mode best practices icon colour is broken.
Bug 1577831 R[Core:IPC]remove `using namespace std;` from IPC glue code
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Changes to 15 bugs - 10 fixed.
1 bugs are not included due to being restricted