This CL sets the default value of the flag wasm_num_compilation_tasks to 10.
The actual number of compilation tasks is the minimum of the flag value and
V8::GetCurrentPlatform()->NumberOfAvailableBackgroundThreads(), which is 8
on my machine.

R=titzer@chromium.org

Review-Url: https://codereview.chromium.org/2054093002
Cr-Commit-Position: refs/heads/master@{#36892}