[coverage] Use optimized builds for coverage data.

Local testing suggests that optimized builds add more speed
without trading off tool usability. We get the following
differences (A: non-optimized build, B: optimized):

Sometimes: Lines instrumented in A (covered and uncovered) are not instrumented in B.
Rarely: Lines instrumented and covered in A are instrumented, but not covered in B.

The latter might simply be caused by timing differences in
the two builds.

BUG=chromium:568949
LOG=n
NOTRY=true

Review URL: https://codereview.chromium.org/1719923002

Cr-Commit-Position: refs/heads/master@{#34193}

build/standalone.gypi

@@ -734,8 +734,7 @@
             'cflags': [ '-fPIC', ],
           }],
           [ 'coverage==1', {
-            'cflags!': [ '-O3', '-O2', '-O1', ],
-            'cflags': [ '-fprofile-arcs', '-ftest-coverage', '-O0'],
+            'cflags': [ '-fprofile-arcs', '-ftest-coverage'],
             'ldflags': [ '-fprofile-arcs'],
           }],
         ],