Add a per-thread proc table using thread local storage

In situations where both dawn_wire and dawn_native are used on separate
threads (Chrome with --single-process or --in-process-gpu), it's
desirable to have a per-thread proc table so that the WebGPU C++ API can
still be used. This eliminates classes of bugs with manual
reference/release errors.

This also changes many of the GetProcs functions to return const
references to the static proc tables known at compile time, instead of a
copy.

Bug: none
Change-Id: I8775bb715b312dd9476a1903fbd797d4b1302614
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/29240
Reviewed-by: Stephen White <senorblanco@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Austin Eng <enga@chromium.org>

src/tests/DawnTest.cpp

@@ -744,12 +744,9 @@ void DawnTestBase::SetUp() {
         clientDesc.serializer = mC2sBuf.get();
 
         mWireClient.reset(new dawn_wire::WireClient(clientDesc));
-        WGPUDevice clientDevice = mWireClient->GetDevice();
-        DawnProcTable clientProcs = dawn_wire::WireClient::GetProcs();
+        cDevice = mWireClient->GetDevice();
+        procs = dawn_wire::client::GetProcs();
         mS2cBuf->SetHandler(mWireClient.get());
-
-        procs = clientProcs;
-        cDevice = clientDevice;
     } else {
         procs = backendProcs;
         cDevice = backendDevice;